Nope. The only person who knows what's going on is HIM.
I can guess what might have happened though. If the two of you haven't been sexual together for very long, he may be feeling anxious or worried about pleasing you. Anxiety is the enemy of good sex. The more you worry, the less you get turned on.
The most important thing you can do is to help him relax and let him know that there's lots of ways to be sexual and sensual that don't require an instant erection. Sex is about much more. Let him know how you like to be touched, and ask him how and where he likes to be touched. Don't have a goal of penis-vagina sex--just relax and enjoy each others' bodies. Unless he has a medical condition or is over-using substances, he'll probably be able to relax, and before he knows it, he'll be turned on. Best of luck to you. Dr. J
there could of been many reasons such as drugs or alcohol .. if those were not involved maybe the partner feels pressured in some sort of way try a more relaxing setting he may have his own personal issues .. be supportive dont put him down because that will just make things worse be patient ! hope the best for you ..