What can i do to help my boyfriend stay hard?

Do him hard. It might be that easy. You be on top and just Do him as hard as you can. Or you can try to start off with a blow job and you really use your tongue a lot to really get him going. If that doesn't work then just use Viagra or Cialis.

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Adding on from the above answer, you could also use "dirty talk". For example; when you feel he isn't as hard as he could be, say something like 'I might have to punish you for being a bad boy, f*ck me harder', etc. It'll motivate him, therefore keeping him hard. Good luck and have fun. Stay safe!