Doggy nightmares

I've had my boy since pup. He 3.5 years old and has always been well treated but almost every night of the last 3 years he will start crying in his sleep. It's a kind of whimper and distressed sounding noise that he never makes during the day, like he's terrified of something happening in his dream.

I always speak to him gently – are you OK? it's ok. Stuff like that and he stops crying immediately.

Does anyone else's dog do this? Is there anything I can do to help him?