I agree with Fish48, he does look a little off. I'm not an expert on mollies but his fins look clamped which is never a good sign (One of my mollies died a week after presenting this sign) and from the pictures his spine looks curved which could mean internal parasite (If I remember correctly). Your setup looks fine, filter and heater but that doesn't always mean the water parameters are good. Did you cycle the tank before adding the fish? I really can't say what is wrong with him and maybe fish48 can help more but I would say the cheapest thing to get which could slightly help is aquarium salt if you don't already have it.
I hope your fish gets better and we're just trying to help you know and be aware that something isn't right which could affect your other fish as well.