Had a similar problem as VC. Managed to get him hard with oral. I asked him if he wanted to have sex, he agreed. After a few minutes inside he went semi hard so was struggling to keep it going. Managed to get him hard again with bj then tried again in a different position. This seemed to help a bit and we were getting a rhythm going but then it started going soft again to my frustration.
I finished with Bj. Don't understand how he stays hard with bj but not with sex.