You mentioned having a heater, but what is the tank temp exactly?
You also mentioned feeding a betta food 3 times a day, but what kind of food and exactly how much?
Do you ever fast? Most people do for one or two days per week - it helps prevent constipation.
How old is he?
Is the weight gain even over his body, or just in the belly area?
Does he have a history of internal parsites, and has he ever been treated with an anti-parasitic food or medication?
Chances are your boy is just bored and chunky. Cutting back on food, and letting him get some exersize by permitting him to flare, chase floating objects, and by re-arranging tank ornamentation weekly to encourage him to explore should all help.