I can speak from experience here. I had the same trouble when I was first getting together with my OH. First of all; please, please, *please* don't be offended by his inability to get erect. It's all too easy to think "he must not be into me, or thinks I'm not attractive", especially if you have any sort of low body confidence (not saying you do). In reality it's most likely the opposite. He probably thinks you're *too* hot!
When I met my OH, I thought "she is the sexiest damn thing I ever laid eyes on", I still think that now. I suffered from my own thoughts and anxieties, thinking "she could have anyone, why would she want me?". I really really liked her, but I was pretty inexperienced too. I didn't want to screw up my chance with her and wanted to be impressive in bed. I tried to read lots of things about how to be better, last longer, give great oral, etc... But when it came down to it, I'd put so much pressure on myself to perform that I was crumbling under my own expectations.
I'm not exactly much of a confident person either so that didn't help. You know how you feel so nervous right before an interview? I was like that; heart pounding, nerves fireing, and I've got this smoking hot body on top of me practically begging for it, and I couldn't get it up if my life depended on it!
When you think about it biologically, it makes sense. It's the fight or flight response to stress. Adrenaline starts to flow, your body redirects blood away from unnecessary functions and to your muscles, ready to run or fight the situation. Unfortunately none of this is useful for love making.
This happend the first few times we tried. It's demoralising enough as it is already, feeling like less of a man, without the added burden of anxiety at the thought that the partner will react badly about it. My OH found it hard to accept it wasn't her fault because of her own self esteem issues. Luckily I was able to talk to her about it and ask her to be patient with me. It definitely helped to know she would try to be understanding. The following times we met up with no expectations of sex took a lot of pressure off. Eventually with time, I felt more relaxed and it came naturally.
But don't be surprised if it happens again though; either not fully erect, or not erect at all. I was a complete beginner when it came to introducing sex toys and bondage in the bedroom, so that was another thing to get used to. I wasn't that I wasn't into it, I find bondage really hot, but I was anxious to begin with. Worrying if I'm doing things right; are the restraints hurting my partner; can she breathe with that ball gag in? Maybe sounds silly to some, but when you have these anxieties, it's hard to switch them off and it just kills your erection.
He'll come around. Just be understanding and supportive. I still struggle sometimes even now if I'm tired, worrying about stuff or just not fully ready. Thing to remember is, not to expect men to be walking talking hard on machines that would have a go at anything with a hole in it. We are humans too. Things will get better. They have for me obviously, since my OH is now my fiancé :)