Can a guy not know when he cums?

So i was riding my boyfriend the other day and I started feeling like he was going a little soft and he was like I’m not sure but I think I might of already came but like how can you not know if you have cum or not?? He said he was enjoying it that much he mustn’t of realised but I thought a guy cumming was the best feeling ever for them so I’m sure he would of known