Reassure him that if he can ejaculate when he masturbates on his own, then he is FINE. If he is unable to ejaculate with a partner at the moment, that is extremely likely to be stress related. You didn't say if this is only a difficulty during oral sex, or if there are or are not problems during manual sex (if you're having manual sex.)
But really, if he can ejaculate on his own, he's physically fine. If he is simply unable to ejaculate with a partner, then two things come to mind:
1) Read the Readiness Checklist: http://scarleteen.com/sexuality/readiness.html
The questions and information in there apply to more than just intercourse, and it's well worth reading. Its possible your partner just isn't totally ready for what you two are participating in, and there's nothing wrong with that.
2) If you're both sure this is what you want and something you'd like to work towards, than you might consider masturbating together as a step towards growing more comfortable with each other.
But you should both be reassured that this is not at ALL an usual situation, and that 99.99% of the time, it's simply a stress related thing. If you can help each other just enjoy being close with no pressure to achieve a goal, it is likely to eventually pass.
Hope this helps,