Hmm, no idea. Maybe it's just the way his metabolism is? I don't know much about mollies, but if they eat bloodworms and
brine shrimp, maybe try to feed him with flakes AND bloodworms and brine shrimp. I feed my fish with a variety of flakes, plus bloodworms and brine shrimp. For example, I will give them 1 kind of flakes today, another kind of flakes tomorrow, then bloodworms on the third day, then flakes again for 2 days, and then brine shrimp. And I repeat the feeding cycle again. I never see any feces hanging from my fish's bellies. When they defecate, the poop usually falls to the bottom of the tank fast.