Ask him about his masturbation like suggested, and also about his BC or pelvic muscles (usually called PC, but BC is more correct for males).
I can masturbate all day if I keep them relaxed, but if I clench them I will trigger ejaculation and orgasm in a short amount of time. Clenching early without strong stimulation may tire them out though and make it harder to do when it's needed though. If he doesn't know how his work, suggest strong and fast stimulation (if that's his thing) and after a bit of that, clenching the muscles and seeing if it leads to orgasm.
That and have him show you a few tricks. I really enjoy hand jobs, but without having a penis some women just don't know how to do it very well. Plus men are different, some may want lube, loose grip, tight grip, full stroke, tip massage, etc.