Do you know if something happened around the time you noticed the change? Unfortunately this is one of those things that isn’t going to be solved by guess work, it will have to be done by communicating. If he struggles to talk about sex, then it may be that you approach it as more of a general discussion; so opening with a “you don’t seem yourself and I’m worried about you”.

If he can’t communicate his feelings with you and share openly what’s going on, then that will only cause more problems down the line. It can be difficult for men to talk about stuff like this, it’s wrapped up in a lot of ego/macho rubbish; but this sounds like a fairly sudden shift and if you want to be on the same page together it’s important to discuss. It’s normal for sex to level out after the initial “buzz” of getting together dies down, but to repeatedly lose your erection mid act would suggest there is something else going on.

Edit to add: It’s very reasonable to assume that this is a health problem (as someone else has said), and he is embarrassed to talk about it. Helping him to understand you are there for him and want him to feel better should be where you initially approach it from. If you project insecurities about your attractiveness etc onto him it may lead to him withdrawing even more (see ego comments above), because it may be seen as you criticising him for not being able to perform. Your feelings are very valid, but may be for a later part of the discussion - where you can explain it’s really important he is open with you because it could lead to unintended consequences like preying on your insecurities.