I would agree that hopefully you can talk your husband into at least making a try at counseling for all of you before he throws in the towel. Discovering your child is autistic can be devastating on so many levels, and I do think it affects the dads differently than the moms. I'm blessed to have a wonderful husband who is a great father, and even he had a difficult time dealing with it. I was struggling with what do we do, how do we approach this, doctors, therapists, etc....dealing with not only my grief but also the overwhelming task of actually figuring out what to do...and he would barely talk about it with me. It took him months - and a few independent verifications - before he would actually believe the diagnosis.
Please ask your husband if he will consider family counseling before going through something as final as a divorce. He has nothing to lose by at least trying it, and everything to gain.
And, should things eventually turn out badly, remember, you and your son WILL make it. There are a lot of resources out there these days for autism, and a lot of progress being made. Your son needs you, and I know you will be there for him. God bless.