It's as if I'm almost there, but it just stops happening. Of course, 5 or 6 seconds (no exaggeration) after my hand gets back in action, I'm there. At first I was thinking it was due to nervousness – but the last few times were almost second-nature, and I wasn't nervous at all. How can I learn to ejaculate with his hand as well as with mine?
There might be something psychological going on, where you're blocking the orgasm because you're subconsciously afraid to have it. From your question, I assume you're able to ejaculate on your own when he's sitting next to you. If so, try to gradually make the transition – you can masturbate yourself while he rubs your scrotum, the base of your penis, etc. Or, you can try wrapping your hand (or hands) around his and doing all the work. In other words, work up to it over the course of several sessions – try to get there by going through some "in between" stages first.