I think DeWe has some really good points re reviewing behaviour/ a behaviour diary and regular snacks.
My eldest has ASD and by the questionnaires we've done with CAMHs hes borderline ADHD. I'd had concerns from day 1 but he was flagged as very different at playgroup when 2.5, nursery in a school at 3, another private nursery at 4 and full time school at 4. The health visitor came out and did an assessment at home, it was a good half hour, she told me he was perfectly normal and healthy and it was just behaviour. He's now 8 and we have had a proper CAMHs assessment for ASD but at the end of the day just because we have a label we still have to deal with what we had to deal with at the start - behaviour. One of the psychologists best bits of advice to me was 'behaviour is behaviour it all needs to be dealt with'.
I find some foods make a difference. If he doesn't eat, which he doesn't if its noisy or he's distracted (like in a school lunch hall), his behaviour is significantly effected. Like wise if he is thirsty, which he doesn't appear to recognise, he gets really crotchety and frustrated. A can of coke on an empty stomach and hes bouncing off the walls - drunk with food its fine. We now have a lot of cooked breakfasts - porridge, pancakes, bacon, egg, sausages and mushrooms, beans on toast - this helps even things out for the morning. The school ensure he drinks milk and has fruit at break. The school ensure he eats a good amount of his lunch.
How does he manage time outs? For my ds they are a god send. He has a time out as soon as we start to see him loose it. Its not a naughty step for him, its a personal space to calm down before things go to far. He sort of recognises (now) that when we ask him to time out its because he needs it. Initially though it was really hard work with kicking, lashing out and lots of nasty things said by him.
You mention your younger one and concerns about copying behaviour. Exposure to other children and a variety of other childrens behaviours can help. Do you have a playgroup you can access at 2.5, to both give you some space and expose your lo to others?