I don't have Facebook (gasp) so I'm curious how you know he's using messenger all day and not just logged in but not actually using it. Can he tell you've been watching his activity? Is he watching you watching him?

What he's doing and who he's talking to isn't really important to your future. Wasting your time, energy and sanity focusing on him and his activity isn't going to bring you to your best life. Besides, it's what abusers hope their victims will do... return the focus onto them so things can continue.

Focus on you, your life and your future.