Re: lasting TOO long?
His problem is called "secondary impotence", which is a problem I have sometimes. Basically, it is his problem but it's nice of you to want to help. Here's a few thoughts:
At some point, of course, you can say "That was wonderful but it's all I can handle tonight". He'll probably understand. Contrary to what some men say, this will not cause him to die.
Men are given lots of socialization that it's rude for us to have an orgasm until the woman has had enough. So, at some point you might try saying "That's enough orgasms for me. I'm just going to relax and let's focus on your turn." Yelling "Come now, baby!" or something like that probably won't help, since he can't, and will just feel more pressured.