Agreed, a position that allows him to "fuck your mouth" like Kira says, might be a good way to learn too. You can always do a kind of trial run where you let him slowly push in to your mouth until you signal him that it's as deep as you can comfortably tolerate so that he has a measure or whatever of what the limit is.
I think because of social gender myths and misperceptions and stereotypes, it's easy to forget that guys can be sexually repressed and insecure, their bodies can be fickle and stubborn and they can have hang-ups and traumas. They can have all the same things that women do about their bodies and sexuality that inhibit them and make it difficult for them to orgasm.
I've been asked, both as a woman and a lesbian, COUNTLESS times for tricks and ideas for how to make a girl orgasm or how to go down on a girl. The answer is, there aren't any. I can't tell you how to go down on a girl. i can tell you what makes /my/ Girl come or what has worked with my exes, but 9 times out of 10, that's not going to help someone else at all. My answer is always, don't ask /me/ how to make YOUR girlfriend come, ask HER!
Several of my exes have had some kind of hang up or issue, especially with oral sex, that i've had to work with them to get them over it. With each one, much more of the work was done through communicating, written or verbal, in person or not, than in bed. (Which bears mentioning: talking to him doesn't have to be done in person. Do it over email or via text or voice chat or IM or write letters or whatever makes you guys both the most at ease.)
Usually, if i can get a girl talking, even one that has tons of hangups or issues or has never been able to orgasm from a certain type of sex, or even never been able to orgasm at all, it won't take me more than one try to get her to come, but ONLY if we've been able to talk about at considerable length. If i end up having sex with a girl i haven't had that kind of communication with, usually at some point i'll have to stop and ask her what she likes, what i can do better or different, or just for her to guide me to help her come.
i have to use totally different techniques with my current Partner than with any of Her predecessors and there's been a learning curve over the 8 months we've been together so far, and i'm still learning! i like to think i'm pretty good at what i do and i have a fair amount of experience in this realm, but even so, it pretty much always takes some communication or instruction for me to make a girl come the first time. And likewise, i've never been with a girl who can make me come without any input from me, and most of my past partners haven't been unskilled or incompetent in any way. It's just that everyone is a little different.
It's hard to get past that hollywood or literary ideal that you can fall into bed with someone and have perfect sex right out of the gate but the reality is that it doesn't usually happen like that. We can learn to practically read our partners minds and be seamless and perfectly in sync with each other and have amazing, mind-blowing, flawless sex, but it takes time and TONS of communication. Once i got past that fantasy, i learned that the best sex is the result of boatloads of communication and a little practice and time.