It sounds like he's stressed in his new home.
Do you have *lots* of plants in there? It could be that he's feeling exposed and nervous about having quite a lot more room than he's ever had before. Lots and lots of plants will *really* help him to feel better, especially plants at the surface. You can put silk plants in upside down (so the bases are actually hanging out of the tank, with the leaves coming from the surface down) so that they float a bit better at the surface, that seems to help my nervous guys the most. By having the surface of their water "covered" they can stop worrying about incoming predators and do more exploration. It looks a little funny, but hopefully he'll settle in and then you can put them in more traditionally later. ;)
As for the food, I've read that it's normal for them to refuse food for 10 to 14 days at first, just because they are getting used to their new environment. What sort of food are you giving? New Life Spectrum has garlic in it, so they will usually eat that pretty readily. :) I've never had any luck with flakes, and my guys don't like the Omega brand (though I know it's high quality)...and if he get's spoiled on blood worms, it can be a fight to switch him over to "real" food later. I'd say give him NLS and he'll eventually eat. :) Also, he may be eating off the bottom. I had one guy who only ate off the "floor" for the first several months. :)
What sort of betta is he? I've got a double tail and an elephant ear who both had some problems swimming when they were first introduced into the new tank. It just took them some time to build up their muscles...and the EE *still* tends to stay at the surface and hide. The poor creature...had I known that the EE's had such trouble I never would have purchased and supported such a detrimental breed subset. :( But, anyway. ;)
Hope that helps at least a little bit! :)