Condom tips?

Howdy so my boyfriend (m20) and I (f22) have been dating for almost a year at this point and had been having unprotected sex the whole time. I’m not on birth control or anything and I’ve been told multiple times that I’m probably infertile, but we all know women’s healthcare has been a bit of a joke recently (I’ve been misdiagnosed/ignored sooo many times I’ve completely lost trust in the industry) so I don’t 100% believe that. I’m still always very anxious about getting pregnant, and the constant anxiety and wondering every month until I get my period has started to take a toll on both me and my boyfriend. We both agreed that it’s time to switch to condoms since I’ve tried just about every method of birth control and all of them have fucked my body and mind up severely.

Here’s the thing though- his penis is extremely small, he’s overweight, and he constantly struggles with getting and maintaining an erection. He also smokes several times a day and is almost always high, which is really affecting his ability to stay hard. When we try condoms, within a few seconds he goes flat and they fall off. We’ve tried different sizes, but the real issue seems to be staying hard enough for it to stay on. I know being overweight can affect your performance and I don’t know if I feel comfortable basically telling him he needs to shed a few pounds so we can have sex. But I feel like that’s something he would totally be willing to work for, so I might try that. I think the best way to describe our situation is that episode of Seinfeld where Susan and George try condoms but by the time he gets the package open he’s lost his erection. We haven’t really had sex lately because of it, and even before we started using condoms we struggled with him going soft multiple times a session. Any tips on how to wear a condom when his Johnson won’t stay up?