To think my child might have behavioural problems?

I'm sorry I'm still stuck on the roof. I know you say it wasn't on your watch but how does anyone leave a 3.5 year old, yet alone one with such behavioural issues as yours, unsupervised long enough to enable him to do this?

The only child I've cared for who tried to do this was a 6 year old autistic child. His aim in life was to escape, but he was never left unsupervised 🫣

Can he go outside and play with practical 'real' things, is your outside environment safe enough for him to play. What I mean by ignore is that it makes such a huge impact on a kid that age to intentionally not give eye contact, physical contact etc for bursts of time. Physically turn your body and face away. The second there's an improvement then refer to it, "I like you are using quiet voice, good" etc but dont say anything else then give a physical deep pressure hug. Google 'heavy work jobs for 4 Yr olds'. Try to avoid meaningless praise... Martial arts are great for little energetic physical risk takers, Karate for kids.

I wasn't meaning to criticise. I was just trying to make suggestions. I do understand how difficult it is with a young child with very challenging behaviour. I have been there. Developmentally they are at an age where they have such big feelings and can't express them.
When he's calm I would try to chat about emotions and give names to some of those feelings so that he can tell you more clearly how he might be feeling. My daughter was better once her vocabulary improved.
I wish you all the best, it is exhausting

Even negative attention can reinforce behaviours you don't want. I'd ignore as much of the little things as poss and try to only make small positive TRUE observations. "You carried the cup carefully, I am proud/I see you ate the banana, I feel happy you are full" it's amazing the difference this can make. He sounds very physical perhaps needing sensory feedback, try a mini Trampette. Can you make an indoor climbing wall with little grips and pegs, he's seeking certain sensations so providing safe alternatives might help. Give him a spade if you've got a garden get him digging holes and filling with water, make a water wall with guttering. He could be really bright and need more technical challenge in his play.

generally I would say the crosser you get with him, even calm cross (if you see what I mean, very stern, very no nonsense) the funnier he finds it. I do find this very challenging. I feel I have no absolute control to fall back on

This is exactly like DS1. The few times I have absolutely screamed at him as he was genuinely close to killing himself or hurting someone else, he has laughed like it’s the funniest thing going. It’s extremely hard.

I would suggest having a clear and predictable routine. Make sure you "cue" what is going to happen and give a warning when you're going out or completing an activity. Also as PP states make sure you give appropriate rewards for good behaviour and ignore bad behaviour when it's safe to do so.
Also I think you are really undermining your own "authority" by pretending to call other people to encourage him to behave. Make it clear to him that you are the one with the boundaries and expectations of his behaviour. It makes small children feel secure to know that someone is ( lovingly ) in charge

Also, what sort of pre school does he attend? When DS1 was at private nursery we never had anything of note handed over. We swapped him to a pre school attached to a primary school. Classes are smaller and the day is more structured than at nursery. It became apparent pretty quickly he was more prone to dysregulation and wild behaviour compared to other children. Again whether this is just an extreme immaturity thing or a true neurodiversity is unclear at the moment.

A lot of this sounds like DS1. He’s just turned 4 and has been this way for just over a year. He has hours/days of extreme wildness, where his behaviour is just out of control. He’ll push every boundary going and do very dangerous things as well. When he’s grounded and regulated, responsive parenting works well and he’s amenable to boundaries. When he’s out of control nothing works. He is also extremely demanding and sensitive to changes in his environment.

Im not sure if he is neurodiverse. It wouldn’t surprise me if he is and it wouldn’t surprise me if he isn’t. As someone said up thread, this isn’t outside of the realms of normal for a 3 or 4 yo. If it still persists at 6 or 7 then it would be pathological. I think, by 3 or 4 some DC have very mature self soothing/self regulating abilities. Some really don’t, like our DC.

DS1 needs a lot of exercise (I’m taking at least 5km on his balance bike per day and a run around at the park or equivalent), lots of food, lots of enforced resting and lots of sleep. He also needs a watertight routine. Do you have these basics nailed down? Despite this, he can still be out of control, but days when he is out of kilter are definitely worse.

Re boundaries and consequences. Punishment in itself is rarely helpful. However, good behaviour should result in a reward - most often this should be your time and interaction. Can include a trip to the park where you play with him (or he plays with others but you are present enough to talk about his fun time on the way home). Every day spend one on one time child led play. If he likes drawing for example let him tell you all
about his picture.
As far as possible ignore his challenging behaviour and model good behaviour. Consequences could be missing out on an activity he was anticipating but replaced with one of lesser value to him. Totally ignoring him as opposed to his behaviour will likely escalate challenging behaviour.
I agree with pp re speaking to HV who may be able to refer you to a Solihull programme if they’re available in your area.

Genuinely? How 'cross' do you get with him? Does he ever get upset after a telling-off or cry? Because if not, and he just laughs at you, then you need to be a lot more forthright. You also need to be absolutely consistent, and never let him get away with things sometimes.

When was the last time you said "DO AS YOU ARE TOLD!!" and insisted that he did what you asked? And then followed it through to the bitter end until he did it?

So secure the area so he can’t get on the roof. If he does anything remotely similar you have a plan for what you will do. Consequences are best when they are immediate and linked logically to the behaviour. You think of what you want him to do instead of the poor behaviour and make that easy and poor behaviour hard.

Please don't use his preschool teachers as the bad guys. We don't want him coming to preschool thinking we're going to tell him off. Probably same for Granddad.

How did he get on the roof before anyone stopped him?

I think you should ring the HV and ask for some advice/support. He may have behavioural issues in which case parenting will he harder or it maybe that you need to change the way you’re parenting because heisn’t responding to it. Not all methods work for all children. I wouldn’t be ‘ringing someone else’ this is like when parents say I will tell the police man. It sends the message that you’re not in charge.

What consequence did you give for climbing on the roof and pouring water on the floor?

Your boundaries and consequences are not firm enough.

Apparently my husband was like this in childhood and he now suspects he had ODD.

childmind.org/guide/quick-guide-to-oppositional-defiant-disorder/

He grew out of it as he grew up and changing schools helped as a teenager. His problem was being exceptionally bright and therefore easily bored and disruptive / disrespectful in dealing with that.

He ended up at Cambridge on a scholarship and is now a partner in a big US law firm - to the surprise of many former teachers! He needed to be pushed academically to relieve his frustrations and once he understood the consequences misbehaving would have on his prospects things got much better.