3 year old started sleeping terribly!

[blondeirishmummy84]

Our 3 year old son was always usually a good sleeper but around the time he turned 3 (8th March) he has been refusing to go to bed unless we lie down beside him til he is fast asleep, and he is coming into out room 3-6 times a night wanting to sleep with us. Sometimes he is whimpering. We are both seriously sleep deprived, worse than having a newborn! We have tried reassuring him all day, winding down for bed 30 mins before with quiet time, taking him back into his bed during the night, but when we leave the room he hits the roof, its like some sort of separation anxiety.
He has also began to have terrible tantrums, which are to be expected. He didn't really have much 'terrible twos' moments so this recent behaviour has hit us like a ton of bricks.
He has delayed speech and language and we start therapy next week. Its hard because he can't tell us what is wrong. Apart from this he is generally a happy little boy and gets on well at daycare, they have no concerns.
He has a younger sister who just turned 2 and he has started being a bit jealous of her, wanting more one to one time with us, which we are making more of an effort to do. They share the same room (we are currently in a 2 bed until we have our house built later this year).
He is in a cot bed with one side down and Im wondering if we should transfer to a single bed or toddler bed?
Some days he doesn't nap but mostly he will have a sleep ranging from 20 mins to an hour. He needs it most days now because he is up so many times in the night and not going to bed until late! Last night he screamed the house down not wanting to go to bed and screamed that much he made himself sick and now has a hoarse throat.
We are just at our wits end and dont know what to do, it has been going on for over a month now and seems a long time for a phase/sleep regression!
Some advice would be great.

That sounds like fear, he's scared of something in the night. It's quite usual for this to develop around the 3rd birthday, so entirely age appropriate.

So it comes down to finding ways to help him feel safe and secure.

At around this age I started leaving the light in in bedrooms. We have dinner switches so it was the main overhead light, but dimmed right down. Highlights don't always produce enough light for a frightened toddler. And also bear in mind that any lower level lamp or if the lamp is in a corner may well produce shadows around the room - and those shadows can look scary to a toddler.

Having something to cuddle may help too. A special teddy for example. Even better if you get an old one (from a charity shop) and give it a back story - This was my teddy when I was little and it always kept me safe so I didn't need to feel scared.

There's no point in language like "You don't need to be scared" because he does feel scared. So instead of just telling him to (try to) ignore his feeling, better instead to find ways to actively reduce the fear.

White noise (all night, maybe a fan?) Can help to stop general house creaking noises from sounding scary.

Then I would stay with him when going to sleep (inc always taking him back in the night). But with a plan to gradually withdraw.

So start with sitting by his bed on the floor. Stay until asleep.
Then sitting on his bed, stay until asleep.
Then stood by his bed, but sit and cuddle him if upset, but go back to standing when he's calm. Stay until asleep.
Then stood a few steps from bed. Stay until asleep.
Then stood by the door. Stay until asleep.
Then stood by the door but keep popping away for a minute or so at a time. But stay close by until asleep. All the time through this always go back and cuddle if he's upset, so he always/trusts you will. But withdraw when calm. He has to trust you'll stay until he's asleep though, so no sneeking away before he is.