Going soft

Brando
I just got into a relationship with an AMAZING guy, he's just a couple years older than me. With that being said, he was a virgin before we started dating, making me more experienced. We have only done the dirty 3 times but each time he would get soft right in the middle.. We're super confused as to why. That's never happened to me before and he wouldn't know why.. I would like to learn more about it and know about how to prevent it or how to just go with it?