I'd echo a lot of Gunsmoke's comments, but add that I would buy cheaper cock and ball rings to start so you can really experiment with size and not waste a lot of money. After having only one other ring (a metal one), I bought an expensive Omega metal ring only to realize (after some more practice and building my confidence) that I can handle something smaller and that this smaller size is MUCH more effective.
Also, while a tight fitting (non-vibrating) cock ring helps a bit in delaying orgasm and otherwise makes the experience better, by far (and I mean "by far") the best thing I've done to help my control is something I've never really heard people talk about: I have numerous sessions in a row (often over a week or two) of sex / masturbating where I enjoy the session (coming close to orgasm many times) but I do not let myself actually orgasm.
When you start the session with the proposition that you can't orgasm, it really helps you focus on the varied pace and depth of thrusts / changes in position that you need to maintain from the start and throughout to engage in intercourse without going beyond the point of control yet giving her the constant stimulation she needs for it to be enjoyable. After awhile, you are able to bring yourself time-and-time-again close to the brink and then pull back without a huge interruption (i.e., a slower, shallower thrust or change in position vs. pulling out completely and abruptly). In addition to learning this control, this process will boost his confidence, which is part of it.
And while everyone would prefer to end their session with an orgasm, it's still very rewarding to have your partner orgasm several times during a session (although we usually need help from the Hitachi Magic Wand on her clit while I'm in her to make this happen though) and it makes that orgasm you finally get to have even more heightened.
Something else on this point that I think is important but rarely discussed is the woman's role in learning the more leisurely, varied but constant pace and stimulation that is needed to make sex last longer. Very often, the women who are complaining that their man doesn't last long enough are the same women who rush through foreplay (b/c they have to get up early in the morning, want to get it over, are worked up in a frenzy already and want to get to the main act, etc.) and are then on top riding him non-stop at a frenetic and deep pace till he finishes or on the bottom allowing him to do this without switching positions periodically or using their hands to slow him up and guide him in a way that increases the likelihood that the sex will last longer yet be enjoyable throughout. You can really help the process by helping set the pace and being in tune to how close he is to orgasm and doing things in position, pace, depth, etc. that slows him back up if he is getting too close to orgasm too fast.
Good luck.