My DS will be ten at the end of this month, he has wanted a sibling for years and just a few days ago said to me "I'm never going to having a brother or sister", when I asked why he replied "because when ladies get old they can't have babies" - I'm 29!
anyways, my DH and I are desperate to tell him but we are going to wait until at least 10 weeks I think (I am 5 just now). We plan on paying for an early ultrasound at 8 weeks so we may or may not tell him at that point. I've got two fears of telling him - How he would cope should happen to the pregnancy, especially as he has autism, and secondly we know as soon as we tell him then others will find out. So for now we are playing it by ear - sometime between 8-12 weeks probably, but we have to be ready to tell others and confident the pregnancy is going well before we would tell him.
My son has autism and coped with becoming a big brother really well. He's a bit younger than your son and was almost 7 when his sister was born. I never told him as such as he wouldn't of understood what I meant as he had limited speech and understanding at that point. We would briefly mention it to him, but he wasn't interested and I didn't want to force the topic. So around the 38 week mark we started getting all the baby things in. So in the car he would say this is my car seat(booster seat), that is baby car seat. This is baby's bed, baby's chair etc, etc. So at that point things started to become real to him. Then when he met his sister he was fine with her, he told me every time she was crying and went to find me, he'd put her nappy in the bin (he had a bin obsession but that is now gone), he'd start wanting milk with her but he was also happy doing his own thing and generally ignores her most of the time. Now as they are older he's helping her play with her toys even if its just for a few minutes, trying to comfort her, playing babies with his own doll, and to top it all off he's getting more fruit in him as he's having Erika's yoghurts as he likes to call them, which is pureed fruit. Can't argue there. You know your son better than me so you'll know what will work with you, but if he shows an interest try teaching him about how baby grows each week or get books to help him adapt to the change.
So with that, I told my daughter but she's only 7 months and I don't think she really cares although she did give me a grumpy look so how knows! With my son, I won't as that is what works for us.