Sex thoughts?

My SO is having trouble keeping it up. He is 25 and healthy with a high sex drive. Sometimes he has trouble performing and we can’t seem to know why. I wanted to ask what you guys thought. I’ve talked to a doctor personally but he seems to write it off that he is probably warn out or that we did it a day ago so maybe it’s just that. As plausible as that sounds I’d like your guy’s opinion! We will be half way through sex and he will just randomly go soft! Never had this problem before until recently so that’s why I’m very critical for ideas!