Sucking on their hand is a developmental milestone/phase. It probably won't last.
My LO won't take a pasifier either
This. At this age, they're discovering themselves and that's part of it. I see it as a plus because it helps them soothe, so they have something right there in the middle of the night to suck if they just want comfort.
Now, if he's in middle a school still sucking his thumb or wrist, that'd be the time to try and kick the habit.