Floppy Cock

My boyfriend is 20 years old and we have been together for four months and we have been having sex for three. Kind of having sex. He can’t exactly get it up and keep it up. Like it will get up if I bite my lip or rub at his pants or kiss his neck, but if we were to try to have sex, it would disappear. We tried natural ED pills and settled for a bottle of my dad’s viagra my mom gave me out of my dad’s cheating stash. It worked and he got it up no problems with the pill, then he stopped doing dip and we were able to do it without the pill. And then it started to flop again. So back on the viagra. We are currently off it and he can get it up if I kiss him and stroke it and then he puts it in right away, but if he slows down, it will get soft. It’s frustrating! Any thoughts? I wanna know what is causing it, part of me is scared that it is because he has a prostate issue and I don’t want that to be true because if someone sticks their fingers up his butt I don’t think he will ever sleep again. Anyone else experience this problem? Any long term solutions besides prescription (for him) viagra pills?