Advice?

So, my fiancé has basically gone through his whole life dreading sex. One of his exes was abusive toward him (I don’t want to go into detail but it was awful.) and the rest never helped him enjoy sex. When he started dating me, it seemed like we were having a lot of fun. It was great. After a while we ran out of steam. Having a baby will do that to you for sure. But we had a deep conversation and basically he said he honestly has no idea what turns him on because his exes never let him enjoy any aspect of sex. He has very low self esteem because of it, and he even has trouble maintaining an erection sometimes. How do I help him? I feel so bad because I have no idea what turns him on, because he kind of shuts down and doesn’t know how to respond. :(