Sorry, but what a pile of bollocks your friend is talking!
Sex is the wild thing you do in the first few years you are together, and when you are making (planned) babies. After that, once reality and real life, the stresses of work and caring for high needs children etc start to take over, sex becomes secondary. It's nice if you both still want to do it, but not a deal breaker if it's not happening. I know of couples who have not had sex for years (for a variety of reasons) and still have very connected, solid, affectionate relationships.
Your husband is making pathetic excuses, and I should know, I've heard every one of them from the father of my children. The truth is, if he had focussed his energies on the needs of the child you both created, perhaps he would A) understand how tired, preoccupied and generally worn out you are, and B) not felt so left-out and isolated. You say you've not been very supportive of your [D]H, but I wonder how supportive of you and your DS he has been? If he's been busy having an affair with an OW for a year, then I'm guessing not very.
He's behaved like a spoilt child, you already have one, genuinely, high needs child, don't take this kind of emotional blackmail from this man, and I use the term loosely.
Go to counselling, use it as a tool to help you separate amicably and get to grips with the fact that this is mainly the result of his inability to deal with life as a family with a high needs child, not your lack of focus on him. But don't you fucking dare put on the high heels and stockings and lure him back to your bed, he doesn't deserve you, and you sure as hell don't deserve him.