It is a submissive behavior. In the wild, it would show submission to other dogs higher in the pack.
Keep him on a leash and redirect him with commands. Praise when he doesn't do it.
The way you talk to him and interact with him should be low key, too. Loud type corrections and interactions only serve to reinforce his submissiveness. Keep your voice firm, but not loud.
My dog is a dominant female and a Lickee.

It's annoying when she's trying to walk and has another dog up her butt, lol.
Dogs.