SillySnail, what do you feed your snail to begin with? That will help answer the question you asked.
Adding a filter shouldn't change how much he eats really, unless what you're feeding him has the risk of being sucked up before he eats it. Remember, any uneaten food, even the wafers, should be removed after a day, as they'll muck up the water if they sit too long.
He should be eating wafers, as said, and fresh veggies whenever you can. Mine loves cabbage, cucumbers, peeled peas and green beans. Most will eat potatos and spinach, all raw of course (Rigel doesn't like potatos, go figure). They like food that is soft, so with the cabbage and beans I let them sit in a cup of tank water for a day and start to decompose before giving it to my snail. That way it doesn't cloud up the water for a day before being eaten.
And yes, they do have a heavy bio-load, but with regular, proper water changes, a small mystery snail is fine in a 2.5 tank with a betta. Keep the water clean and watch for clouding.