Have you tried a cock ring? Little stretchy ring that goes around the base and is meant to help slow bloodflow to make a guy last longer/maintain erection longer. You can get them at sex shops, you can get vibrating ones too that are designed to hit your clit and stimulate you more as well. Make sure you clean it if it's reusable.https://en.m.wikipedia.org/wiki/Cock_ring

Cock ring, if he's cool with that.