Red rocket

My older dog keeps getting red rockets in his sleep. And not just once or here and there. Almost every time he falls asleep lately. He humps the air and everything. I know it's natural for them, but is it natural thus often?? Is there I can get him to stop? It's starting to bug me because I honestly just find it gross.

And yes, he is neutered. It doesn't stop the natural instinct or take away the penis. He just no longer ejaculates.