Whoa. Not good. Definitely fast him for several days. Unfortunately, not long ago someone posted about a betta who was overfed (mother put entire cube of frozen brine shrimp in) and the betta passed away. :( I really hope this one has a happier outcome.
I'm not sure but as a precaution, you may want to put him in a barebottom QT container and start an epsom salt treatment. Since pellets tend to expand, too, and he's had a bunch of them, he could be facing some serious blockage in his near future. Take a 1g jug or something like that and mix 1-2 tsp of epsom salt in it, pour that into his QT container. You can float the container in his original tank so he stays warm (bettas need temps of 78-80 degrees farenheit so if you don't have a heater yet, pick one up soon!).
I really hope the fasting works. Keep us posted.
The other advantage of the barebottom QT is you can watch for poo.