So first things first, stop worrying. I'm going to talk about myself for a bit and then apply it to your situation. I (m24) have a number of kinks but my biggest kink is getting my gf(23) off. It can get silly, I will last for a long time until she gets there just because I prefer her getting there to myself. A frequent occurrence is me not climaxing but making sure she gets there anyway she can. She does not understand me, and most people wouldn't complain as it's all about them, but she has explained to me that because she cares about me she cares whether I'm enjoying myself. Honest to whatever deity is out there, I get more enjoyment from getting her off most of the time than getting off myself. That's not to say there aren't times where i need it but they are usually few and far between.

Now I've never used the excuse of "storing my sexual energy", and to be honest that sounds a little far fetched but I can tell you that there are men out there that act this way, me being one of them. Another explanation that I can think of is that he is desensitised, this may seem far fetched but he could be struggling to get off because he's into something you're not? Desensitised by porn? Used to finishing himself? (I know you said he doesn't currently watch porn or masturbate but doesn't mean that he hasn't in the past). There are a number of reasons or it could be that he just gets his fulfillment from your enjoyment, it sounds far fetched but i can testify it happens.

The best thing you can do is talk it through with him, why is he "storing his sexual energy"? (sounds like something sci-fi-ish) Is it something to do with what he likes? Is there anything you can do to help get him there? It also comes down to how open and trusting you are, you can sit him down and talk to him but do you trust what he is telling you is the truth? If so congratulations, you are going to be a very happy person for the rest of your relationship.

N.B. Not all blokes are similar.