My boyfriend lost his errection during sex for the first time

My bofriend and i have been dating for over two years i see him three times a week. We have sex now and again but always have forplay when ever we see each other. My boyfriend has never had a problem with keeping an erection, however when he has had too much to drink he finds is hard which is normal.

Last night we were having forplay as usual and he came errect, then we went into the shower and did some more forplay and he still was errect, but when it came to having sex, i got on top and all was fine when suddenly he went limp, it has never happened before, do u think he has something wrong or has he gone off me?

He had done a 6 hour shift but said he wasn't tierd, we had a few drinks but not enough to have an effect?

He smokes and drinks, dosn't excercise much and dosn't eat that healthy, he isn't overweight

Please can you help me