Intimacy and trying for children.

My fiancé is autistic but he went a long time not knowing. 

It was my knowledge and work with children  with autism that made me pick up on his very autistic traits. He got diagnosed 4 years ago. 

We work nornally very well together he tells me often that he's grateful that I'm educated and know how best to support him. 

The one thing i just don't know how to move forward with is our sex life. We've been together for 6 years and we've both been through so much. I dont want to be too graffic here so im going to try and be straight to the point. In 6 years we've only successfully made love once. We both want to have children but i don't know if we can. Sex used to be painful for my partner until he had a circumcision. He wanted it done so that we could try for children. Since then he's no longer in pain which is good but whenever we try he looses his errection and always has done. Bar one time after his circumcision, every time since then we've failed. When we've sat down and spoken about  it, he tells me that he wants it and he tries to think of all the things that make him aroused but then his brain fogs out and he goes down and takes him ages to get back into it. If intercourse isn't involved he will stay errect and ejaculate with no problems at all. For a long time i thought it was me. Until he tried to tell me what goes on in his head. I feel really alone in this and i wondered if anyone else with autism and a partner struggles with this too and if there is any advice anyone can give me or to even guide me in a way that could best help him. He tells me all the time that he'd love nothing more than to have his own child with me. But unless we move past this we never will and it breaks my heart, I'd love nothing more than to become a parent with the love of my life. It hurts every day thinking that i Will never become a mom, never know what it feels like to feel a mothers love and the same emotions for my partner. Neither of us know what to do to make this better. So if any of you lovely human beings can even just relate even a little, you'd be helping us so much