Hello saskia: I see this is ysour first post here on PC. So... welcome to PsychCentral!

I'm sorry we are only now replying to your first post. Please forgive us.
I don't know as there is a lot I can offer with regard to this.

Erectile dysfunction is, in my experience, one of those types of problems that tends to feed on itself. The more difficulty a guy has with it, the more trouble it causes him. It becomes sort-of a vicious circle, so to speak.
I'm certainly no expert with regard to any of this. So this is just my personal opinion. But from my perspective, I think your bf is probably correct in that this is not a relationship issue. He's not having the problems he's having because there are problems in your relationship. It's a matter of the medications he's taking plus the emotional distress his ED problems are causing him.

So, at least to me, him going to see a counselor or therapist by himself makes sense. (And kudos to him for being willing to do that. A lot of guys probably wouldn't.)
Now having written that, the difficulties your bf is having do have an impact on your relationship as a whole. So it certainly may make sense for the two of you to participate in some couples counseling at some point. But again, at least in my personal non-professional opinion, the two things should perhaps be addressed separately. Let your bf work on his ED issues with his prescribing physician & perhaps a therapist. Then, perhaps a bit later on, consider going together for some couples counseling to work on your relationship as a whole. At least these are my thoughts with regard to your post. My best wishes to you both.