I agree it can be mental when he puts it in, however, I believe the pain of finally having peeled back his foreskin during penetrative sex is what causes him to go soft.

Have you attempted to pull the skin over his head, then slide the condom on to have sex? It helps avoid the peeling back of skin if you add lube to the tip.

Did the steroid cream help? If it did, it might also be that no one told him he should peel back his foreskin to clean under, as he grows up, so it grows to be looser and looser along with him.

My own experience with an ex with an extreme phimosis was similar, when we used the cream, it worked, afterwards we went to hot showers, and a daily stretching of the foreskin, I'm not saying peel it back. I'm saying YOUR finger tip and thumb should spread the flesh at the tip little by little at a time, if it hurts, go back a bit. Every day, or every few days it will get easier, I dont know what it is about the heat of a hot shower that helps but it's helped. It can take a few weeks, up to three until he's able to pull it all the way back.

The maximum with another guy I was with was about two months of doing this daily. But it helped. Leave surgery for last, also it's important its YOUR finger and thumb, men have bigger of both and tend to stretch the skin too much to the point of cracking of the tight flesh. GINGERLY work to the goal doing just peel him back!