Whether or not he has AS, you are not over reacting as sending a child home from school at that age is not normal, whatever the reason. Its actually an unlawful exclusion (see IPSEA). You need to take time to get your head round it, realise his problems now recognised can be successfully addressed, and find out more. You can borrow lots of books for free from Cerebra. Don't be put off by the nature of the charity, its a fab scheme where they lend out books for free. You also need time to cry, scream and maybe grieve. However none of this should stop you rolling up your sleeves and going into school tomorrow and saying 'this is what I have been reading about / worried about, now we need to find ways to help him'. They may be relieved you are thinking on these lines.
You need to say that they can't keep sending him home. If school don't know what to do then there are support services they can call in who do eg behaviour support, autism outreach, ed psychs. The school can also fund a 1:1 TA from their own funds (all mainstream schools are given funds by councils to do just this)
If it is AS or similar they cannot apply the same rules to him eg aggression cannot mean exclusion. It is their job to reduce the aggression so he can access learning.
They need to start writing down what happens before the lashing out, and what happens after. Was there a trigger? Or as DW says is it the fact he gets to go home which is the trigger and therefore their response could actually be making the situation worse.
Reward schemes can work but need to be immediate and based mainly on positive approaches eg if he behaves well for x mins he earns 1 min computer time, or whatever reward is motivating for him (which may not be stickers, smiley faces etc but may be to play with the train or lego or computer). To start with you might be rewarding him every few seconds etc and say have a token board with 10 spaces to fill and then when he gets to 10 he gets to play. Then come back and start again and start spacing the tokens out. You might start with a token board with 9 points already earnt and he just has to earn the last one and then get the reward to get the idea. It really depends what level he is at. This is why he needs 1:1. A teacher with 29 other children does not have time to give him this individual attention.
Nothing that is done for children with AS or ASD will be harmful. So you can start using approaches and see if they work whether he turns out to have AS or not.