that is a possibility, but i wonder since as long as he's on top he can orgasm fine...
if sex is pretty new to him, it may just be that he hasn't yet gotten past the common stage of "finish fast or wander off mentally". nothing wrong with that as long as he keeps growing as a person & partner, nothing at all.
i think a consideration here is mostly the positions... when he's on top & controlling the speed/style more than her he can move more easily in the way *he* reaches orgasm easiest in... which often isn't the same way as what gets one's partner fully aroused & orgasming.
and vice versa, when she is on top, she is controlling the motion more & therefore that just-right stimulation may not be as intense for him -thought still pleasurable- and he's losing his erection due to it.
this happens even with partners who have been much experienced w/others & then w/each other for years. two people just don't get the same amount of stimulation from different positions, though they can still be fully enjoying themselves.
women and men also tend to run at different time rates often when it comes to reaching orgasm. women often need more foreplay time than men & simple intercourse for most women isn't enough to achieve orgasm easily (not all are the same of course, this is a just a summary).
if he's concerned about not pleasing her, first he needs to talk to her clearly about his concerns about this. he needs to talk to her about their sex, the fact that he's feeling a bit uncomfortable with the experience difference, and learn from her what *she* feels they need to do to fix this together.
orgasm shouldn't necessarily be the end-all be-all of sex, but both partners do need to end each encounter feeling good about it. there are lots of things within their private sex life that they can do to help them both orgasm. and there's also no reason that the act of sexual intercourse needs to cause the orgasm for each. if he finishes before her, he can stop intercourse but continue other forms of sexual contact. his losing his erection doesn't need to be the end of sex, it can always be regained during other acts. or they can change positions or who's controlling the motion primarily if one or the other needs additional stimulation... and tons of other things.
but everything i've said so far really comes down to communication. -whether it's talking about it or doing it, they need to communicate concers, needs & wants to each other.