Hi Fishtastic!
Yep, Nick's correct. Your molly is most definitely a male. I think he's beautiful, too! Thanks for the pictures.
Your molly may be a little constipated, which is common when they are fed freeze-dried foods. If you thaw 1 frozen pea, cut out a tiny piece the size of his eye, and feed him just the thawed piece of pea twice a day for a few days, he'll look less pregnant.

Mollies actually need vegetable flakes... much more so than shrimp or blood worms. Walmart sells veggie flakes in their fish section.
Your tank is a bit small for him, so you definitely don't want to put any more fish in with him, not even a small one. Your fish will be much happier and healthier if you change about a gallon of water every day, so he doesn't suffer from poor water conditions. Don't forget to add just enough water conditioner to clean the newly added water.
Best of luck!!