Tell me if I'm wrong...
okay,
I'm 23yo, I'm good looking, and personaly, I NEVER had any problem like this (satisfying girls, getting erection, anything) in my whole life... Since I started dating this girl I'm REALLY interest-in about 2 month ago... I had all kind of problem, and I know it's all in my head.. (and I'm very attracted by her, there is no dough about it, she's not the problem. so don't say a word Badmotorfinger, you show us all you don't know what your talking about). I know I just have to relax, stop asking myself: "what she think of this?, what if she don't like that?, is she attracted by me?, does she want a longtime relation with me like I do with her?". I know this because anytime I just enjoyed moments in good company, not having tones of question in my head at the same time, I had no problem at all. And like you said, she think she's the problem what's make it even worst. That's very frustrating not being in control of your body in those time (specially when it never hapened befor and you don't know what's going on, and on top of that you have explanation to give)... but luckily for me, it getting better each time i'm with her without questioning myself for a sec, and enjoying what I know will be the best moments in my life, moments I don't want to pass by or forget, no matter what happen next.
With all what you wrote in your preview posts, I felt a bit like he's in the same situation, so I think this could be the problem specially if he's depressed, stressed, not self-confident... I'm going better and better because I understood my problem, and I've been able to work on myself mentally and put what upset me aside (and thats because I never had these problem befor, I'm self-confident, and I know what I'm made of). Otherwise, I could eazily get Stuck in this situation...
The thing you could do is take some times together relaxing, getting yourselfs comfortable, getting closer, getting romantic, without thinking about anything outside your "little bubble" (not even about sexy-time), enjoy good time... things like a romantic candle light dinner, bubble bath together (every one know what a good, Hot, relaxing bubble bath in the dark with candle all over the place can do!)... drinking a little can be a huge turn on and help puting discomfort, issue, or work probem aside, so it not a bad idea too! I think wiping all what disturb him out is the key. I'm sure it's just mental.
Personaly, I don't think that being told by a girl to go see a psychiatrist or a Doctor would help me get back to normal, but i'm not in the exact same situation so to see a professional could be what he need. If it's was a physical problem, i think he would have already saw other symptoms, but i'm not an expert.
(...And no offence but, i'm sure you can leave story about his mother or mutilated small animals out of this... it's not like if you caught him naked in the corner of a room eating a raw steak... I don't think you have to get to this point.
(P.S. I'm french, so sorry if my English is a bit wrong)