Pre-sets are not known for their quality and accuracy--if it's not heating the water enough you should definitely take it back. You should get a 50watt heater with an adjustable temperature dial. I use Hydor Theos, but Marineland Stealth and Marineland visitherms are other good options. The good thing about the stealths is that they aren't glass, so they take a lot of abuse--but unlike the Visitherm and Theo they lack the indicator light, which I prefer to have. After all, it's a heater, not a baseball bat--it's ok with me if it's made from shatterproof glass. :) You can see them/order one here: http://www.fosterandsmithaquatics.co...ps/c/3578/3743
if you don't like ordering them online and they aren't at the fish store, you should try asking them to special order one for you. Most stores are willing to do this.
On another note, I would not feed him anything unless you see him poop or you see the bloating go down. It would be best if you could actually find the poop--but that isn't always possible. I know it seems like you're starving the fish, but it actually is doing more harm than good to stuff more material in his intestines than he can handle. Fish can go for weeks without food, so don't worry. If you feed him anything, frozen daphnia is a wonderful source of fiber, in my experience it works just as well or better than the blanched pea treatment, and has the added bonus of providing your fish with nutrients they can actually digest instead of just veggie matter.
If you have some frozen (not canned), unsalted peas in your freezer, that will work too. What I do is dip a little piece of a paper towel in some tank water, wrap a single pea in it, and put it in the microwave for a few seconds. Allow to cool, then remove the shell/skin of the pea--you will see that inside there are two halves. Moosh them up a bit, and then take a tiny piece, comparable to the size of one betta pellet, and feed it to the fish. After this, he gets no other food until he poops. This is like--fish metamucil.
Has he had any freeze dried foods? These are notorious for causing constipation. If you're just using pellets, remember to soak them in a little tank water before feeding them to your betta. Bettas weren't made to eat bready air-filled foods, so eating a dry pellet for them is kinda like us eating uncooked rice. Very uncomfortable. More heat should also help speed things along.