My boyfriend has burning sensation while pissing after sex

So need y’alls opinion. My boyfriend says that it’s not very rare but occasionally after we have sex he goes to piss and it burns but I’ve been checked out and I am healthy and clean. I don’t know what is going on with him or if it’s me. But he is in pain and I feel useless since I can’t really do anything but give him water pills to piss out what ever it is or give him UTI medicine even tho I don’t have a UTI and if he had it wouldn’t I have it? We’ve been together for almost 5years and he says it’s only happened 3x’s the whole time we’ve been together so please help me out!! Any advice or suggestions for a cure I’ll take them!!!