glassescase I kind of understand the question you were trying to ask and hope it just came across badly - but honestly - you really don't have a clue what you are talking about with regard to having a SN child and further dcs.
My ds1 has AS and he was an absolutely amazing, compliant, angelic child - until he went to junior school, when his world changed significantly, both socially and academically and he just couldn't hold it together any longer.
He is now 9 - the same age as the OP's ds and we are currently going through a very rough patch with him, as he tries to cope with and understand the way he is getting left behind by his peers and how the social side of his life is constantly shifting. Its very hard for him not being able to have the same freedoms as his peers and is something he pushes against, constantly testing the boundaries etc.
At school the work has become more complex, involving abstract concepts that he isn't equipped to deal with and as a result his self-esteem has taken a huge knock - going from one of the highest achievers in his year right through infants to zero progress over the past two years.
I have two other dcs, one from before we knew ds had ASD and one two year old who has brought so much love and joy into our lives - and in particular to ds1 - that it would never have been the right decision not to have her.
OP - I feel for you. Its a tough age for children with ASD and life at it can be a very bumpy ride for both them and their families.
I agree with the posters who've suggested trying to organise a routine and strong boundaries, with rules you can agree with him. If he has an obsession at the moment, use it to reward him - my ds can earn an extra 5 minutes on his beloved nintendo ds for making good choices and doing things without arguing etc.
Routine is particularly useful in the holidays as it helps them to cope if they have some structure to the day. We tend to do getting up and breakfast at the same time and in the same way, which starts the day off well, then there are certain times throughout the day where we do particular thing, eg he is allowed an hour on his ds at 1.30pm (with the caveat that if we are going out he can have it earlier) we do snacks at 10.30 and 3.30 (same times as school days) and have tea at the same time. I also do some very basic schoolwork with him at the same time every day.
As for sleeping - my ds's sleep was terrible until I put him on a good quality omega oil. We use a vegetarian one, but there are lots of them on the market. I also learned not to stress about how late he was awake and bought him a booklight so that if he was awake later than his brother (they share a room) he could read. (His rules include not getting up before 7.30 am and no noise after 8.00 pm.)
I would recommend a GP appointment and/or a chat with your health visitor as a starting point for getting some support. LauraIngles, suggestion for calling your local Surestart is also a good idea and you should be able to call your Council's Local Education Authority to find out about any Autism Outreach or Inclusion services that could help and they in turn will be able to advise you about any support groups that you might like to get involved with.
Finally, please do come over to MN Special Needs. There are lots of us over there who have children with ASD and it really does help to talk to others in the same position.