Formal exclusion means they should give you a letter saying why he has been excluded. If they ask you to pick him up insist on being given the letter before you take him home. Otherwise they don't record this as exclusion.
What sort of behaviours? Ask them to do abc chart for every behaviour A = antecedent (what happened just before) B= behaviour (what did he do) C= consequence (what did they do). Then see if any patterns. Sometimes it's c which causes b (if c is you get sent outside some kids will do b to get c to happen).
Also get them to try giving him regular breaks during lessons DS earns tokens and gets regular breaks / rewards eg to go read a book in corner or go on computer for 5 mins, then come back and work again.
They need to catch and reward the good behaviour and as much as possible (provided no one is harmed) ignore the bad. So they need to reward before things go wrong. That might be every few mins to start.
Worth reminding school break time is not a break for our kids, it's work, and often the hardest bit of the day, so they need to build in downtime during lessons or allow him to have quiet time during breaks. That might stop things escalating.
Is there a behaviour outreach team? They can sometimes be better than autism outreach at putting in place behaviour plan. A behaviour plan should be positive he knows the rules, earns points for keeping to the rules, gets reward. Does he have any access to TA who could run a token / reward system like this (often called token economy if you want to look it up). The tokens should be targeted at what want to work on so if it's staying calm he should get tokens and praise when he's calm and they should say why he's getting the tokens so it's really clear eg you stayed really calm when... Here's a token etc.
DS has ABA in school and his behaviour in school is really good because we have put measures in place and he has FT 1:1; but I've seen several other children without ABA whose behaviour is awful because the staff don't know about autism, don't know what to do etc. If he is at risk exclusion it's because the staff are not managing it properly / are inadequately trained.
If there are particular situations when behaviour happens eg assembly, when it's noisy etc then they may have to withdraw him and build up his tolerance slowly eg he just goes into assembly for last minute, then 2 mins etc (and gets tokens and praise every few seconds for keeping it together).
We've just moved to a school where a child left (his parents moved him to SS before he was excluded). I only found this out after DS started. I am sure seeing the ABA staff work with DS the staff are thinking if the other boy had had the same level of support and specialist expertise it would not have got to that point. School staff are not trained in how to deal with behaviour (they often get it wrong with NT kids too by not being consistent enough), so you need to push school to call in support eg outreach.
It is really really common for SS to start filling up from yr 3 with kids whose behaviour has not been managed in mainstream properly. Here there are very bright kids in LD schools for this reason because mainstream staff didn't know what to do and there was nowhere else for them to go. But ABA approaches can really turn things around. You might even want to think about private ABA advice if that's an option.