could be he's just not use to the new enviornment yet, i'd give him another couple days and see how he's acting. alot of times when u add fihs into a new setting they become stressed and it can take a little while for them to calm down.
sometimes fish will also catch ill after added to a new tank because the level of stress can drop their immune system. keep an eye on him and make sure he doesn't develop ich or any other parasite/fungal infection.
what have u been feeding ur mollys? u wanna make sure they have a well balanced diet so they are healthy and strong. this way they don't succumb to disease, will be much more active and will give birth to healthier fry. i feed my platys and swordtails flake food, tubifex worms and blood worms a couple times daily and some sort of blanched green vegetable matter (i.e. spinach) once a week to help with digestion.
