Is he too excited or what?

It's not so much the speed, but how far off his penis you go . Just try not to let it come all the way out when you are riding him, and it should be just fine .

If you make the stroke too long, it can come out, but since you said "flopping", that sounds more like he's not fully hard. Some guys don't get rock hard, and for them, this is always a problem. Still, slowing down in either case may be good.

It's got nothing to do with rhythm or slowing down. That's just what it is. Really really good sex. That's why he is going to hard and fast. But slowing down does help. And just tell him not to thrust so long. Short burst seem to work best.