That act alone probably means nothing, but context is important here.
For one, it depends on the type of comments he's leaving. There's a difference between being friendly and flirting, obviously. If he posts "HAWT!" on a bikini picture of her, then yeah, that's not good. If he writes things like, "Congrats," or "Happy birthday" or "I thought you hated sushi," I don't see the big deal.
Is he showing interest in her in other ways? Does he talk about her all the time? Or have they simply remained friends since their breakup (which is rare, but it does happen)? You have to look at the big picture. Leaving Facebook comments alone isn't evidence of a problem.
If it bothers you, you could ask him to stop. He'll tell you they're just friends, but if he knows you don't like it, maybe he'll back off a little.
Thanks for the question.