Ever Had This Happen?

We're newlyweds, a month in. Enjoying each other regularly. But some nights, for whatever reason, we're going at it and he is clearly enjoying, and then he goes soft. He is very sure he didn't finish... but he just loses the erection. Happened again tonight.. maybe the 3rd time so far. 
He mentioned earlier that he noticed after going to the bathroom that he was dehydrated, so maybe it's just that? Anyone have the same issue? He's a heavy guy so that may also play a factor... but other nights have gone just fine.