My boyfriend is impotent - what should I do?

This is a candid discussion about gay sex - you've been warned.

I've been in a 3.5 year relationship with my boyfriend (we're a gay male couple) and our sex life on the whole is pretty good. We've been having less sex recently, but when we do have it, it's great.

However, since the beginning of our relationship, my boyfriend has been impotent. He sometimes gets hard, but it doesn't last long and it's never hard enough for him to take the active role in sex. Lately, this has been bothering me more and more. I'm versatile in bed and sometimes I love to be dominated and, to put it bluntly, fucked. The orgasms are amazing and it's nice to have your man take charge. But he can't do that. And it's not just about the *******. I can't suck him off either, because he never gets fully hard. There's something so sexy about seeing a gorgeous man with a big erection and I really, really miss that part of my sex life.

As a result, I feel like I'm missing something. I've spoken to him about it in the past, and I've basically said to him what I've said here (in nicer terms)... but unfortunately, his reaction was to get really angry and yell at me, saying things like 'how do you think this makes me feel?' and 'why don't you just go and **** someone else?'

I didn't want to hurt him and I felt awful for damaging his pride. I understand his frustration and know it's probably even harder for him than it is for me (no pun intended). But I wanted to be honest with him, because that's what relationships are about. However, after that... not so much. I don't want to deal with that argument again. Over the years, he has told me time and time again that he'll "go to the doctor" about his impotence, but he keeps putting it off. Probably because of pride/embarrassment.

Another thing that worries me is that he seems to think his impotence is a result of using a rubber band as a cock ring several years ago, which was wrapped too tightly and caused him immense pain afterwards for a few weeks. I worry that he's done irreparable damage and wish he would speak to a doctor.

So I'm a bit stuck for solutions. I don't want to have sex with other guys (with or without his permission) and I don't want threesomes or anything like that. I just want him to be dominant in bed sometimes. I know it sounds like a small problem, but honestly it really concerns me that this is it for the rest of my life and I'll never be completely satisfied sexually.

How do I get him to deal with this problem without damaging his ego/risking our relationship? How can I get him to finally go to the doctor?
What if he does try to sort out the problem but nothing works and we continue as we are? I just don't know what to do. Please help!