Bring up the idea of using sex toys. Don't jump to like a dildo or vibrator, Start small, like a vibrating cock ring. Something that you can phrase as being for the both of you guys (which it would be) That way his confidence shouldn't be too harmed or damaged by it. If stamina is an issue, I would recommend getting a cock ring that can stretch around his testicles. As it could help him hold on for a bit longer.