Below a guy's dick but before the ass is the muscle that controls orgasm.

As he's about to cum push it in pretty hard (you should easily be able to tell how hard is necessary, it's really not that hard at all). He'll orgasm but no cum will come out. Then if you continue sexual pleasure he will be able to not get soft before getting hard again.

When you do this however, all the chemical reactions that result from sex do happen so fatigue may set in (as that is all related to the brain). But if you're pleasing him sexually I highly doubt this will matter!