Why is there a loss of erection while penetrating during sexual intercourse?

Hello doctor,

I am in a relationship with a really great guy. We started having sex in the past month and are having some issues. He has no trouble getting an erection, but it stops once he is inside me. He is totally into me, tells me I am not doing anything wrong, and is extremely attracted to me. This is not a problem at all when I am going down on him but just with penetration. He said he has never had this issue in his life, and he is beyond frustrated and does not know what to do. If it is not a physical issue, what can he do to clear his head mentally? We are nervous about me getting pregnant as my cycles are not regular, and I thought that might be affecting him. I am at a loss as to know how to encourage or help him, and I am afraid it is going to severely affect our relationship. I am as supportive as possible and never pressure him, but I can only imagine how emasculating this must feel for him. Any advice you could provide would be greatly appreciated.