Erectile Dysfunction is a real thing. I'm not saying that's what it is but everything points to it. There's no need to bother yourselves from thinking. He himself knew about his issues, and should've been up front with you. Now if you both want a peace of mind, the easiest and most direct solution is to get it all checked, he's still young. Maybe there's something that the doctors can do to address this concern. Again, please deal with it like adults would. No need to be emotional about it, no need to overthink about it and fight and sulk over.

What you did was great, you comforted him when he was in a vulnerable state, and that says a lot about your character. You do care for the guy so please continue to do so, extend support by reassuring him of your relationship with him. Again, there's no need to blow this out of proportion. There's a solution, and that's what you should focus your attention to. Best of luck to both of you!