I would wait awhile until you know how serious your ex is about "sticking around" this time. Perhaps require him to meet with your son's psychologist or do some reading on AS. Perhaps you should meet with him a few times to fill him in on what has transpired in their lives while he's been gone.
In the meanwhile, you need to bring the topic up with your children. I would broach it carefully. Let them know the story and what transpired leading up to where you are now (I am a firm believer in honesty with children or else they'll find out the truth later and resent you for not telling them). Let them know he is wanting to meet with them and find out honestly how they feel about this.
Now, given all this I do believe in a parent's right to see their children as long as they are mentally and emotionally healthy enough (both the parent and the children) to deal with it. However, when you have a child with AS, it is a bit more complicated and it may take longer before the meeting would be appropriate.
As far as your current significant other, well he deserves some respect also as he has been the day to day caregiver around these kids. Talk with him honestly. Show him there is no danger of your ex coming into your heart again. Let him be involved in the discussions if you feel it would help. Sounds like you have a lot of hard, honest, emotional discussion ahead of you.
Good luck.