I have to say if your heart is not there, don't stay with him for your child. My last ex boyfriend had twins and he and his wife split when they were 3...he and I went out when they 9-10 years old and they were AWESOME, happy children...both he and his ex had been and still were very careful to provide a stable, happy environment and were even able to be "friend-like" with one another post divorce and as the years had gone on. So I think communication with your ex (should you choose that path) re:care for your son is very important.
Also, my then boyfriend's ex-wife remarried and he and their stepdad would go to football games sometimes, etc. The situation turned out very well because everyone took care of their own needs, but also put the kids first as they had been growing up. I think you can do the same thing, even with an autistic child...he just might need more love and attention, but I bet as a happier person and by being true to yourself, you can do this.