Nothing's going on. This is his body saying "I've had enough".
Men have a refractory period after ejaculation; this may be anywhere from a few hours to a few minutes, to even a whole day. But during that time they are physically unable to orgasm. When you push that cycle over an over again, even if your partner refractory period is short (which it appears to be; 5 orgasms in one day is pushing it a little) you're going to get to a point where his body's going to say "Alright, I'm done".
Copyright 1998, 2014 Heather Corinna/Scarleteen
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