i agree with Lupin, any fish needs a varied diet, just like us human beans, lol. what your betta has done is trained you to only feed him bloodworms, cause thats what he knows he likes. mine would give me the betta equivalent of the puppy dog face when i put 'boring' food in their tanks all the time, lol.
i'd fast the fish for a couple of days if he's overly plump, it won't hurt him any. on the day after fasting, offer some different food, such as flake or betta pellet. just give a little to see if he'll eat it, that way the water won't be fouled by leftover food.
as for the smooshed pea, i've never fed my bettas peas, so i can't comment on that remedy. none of my bettas ever got fat or constipated.
sounds like this lil guy has been eating a tad too heartily. bettas don't need to eat a whole lot. a small pinch of flake, 4 or 5 pellets, or about that same amount of bloodworms, is all he really needs per meal. and a once a day feeding is enough for bettas, since they can be little gluttons. many fishkeepers will fast their fish for one day a week. i don't personally do this, but it could be beneficial in this instance.
one other thing that is a little concerning is your feeding of mosquito larvae that you find outside. i wouldn't do this unless you can be 100% assured that these larvae are free of pesticides and other contaminants, which is difficult to determine in an urban area. i'd stop feeding the larvae that you are catching outside.
another thing i thought of is this betta's living quarters. 2.5 gallons is rather small. if you can upgrade him to a 10 gallon he'd have more room to swim around and that would probably also solve the chubbyness issue. when i moved mine from bowls to 10 gallon tanks, they were much more active.
