My bettas always like to nibble on the algae wafers and shrimp pellets that dissolve on the bottom of the tank. If you want to feed him something besides the bloodworms, try a bit of dissolved algae wafer. (I would not drop it in dry; it can cause digestive problems if he eats too much and it swells inside of him.)
I would go with the 10 gal because they are cheaper than the 5's and for less $$ you can make your betta much happier. I have a betta in each of my 10 gals where they have floating plants and rock caves to rest/hide in. Their tank mates have included a female betta (some males are more aggressive than others and this may not work with yours), harlequin and brilliant rasboras (never any problems there), a dwarf guorami (had to introduce them to eachother very carefully...started with a tank divider for a month and then rearranged the tank decor just before removing it, but now they get along fine), mollies, platys, danios, and snails. If you want some tank mates for him the rasboras (a small school of 7 or so) would work nicely. They are a nice, quick contrast to the slower moving betta and they don't fight or reproduce constantly. He won't pick on them either. One larger "Mystery" type snail would be fun, too, and would help clean the tank.
I would go with the 10 gal because they are cheaper than the 5's and for less $$ you can make your betta much happier. I have a betta in each of my 10 gals where they have floating plants and rock caves to rest/hide in. Their tank mates have included a female betta (some males are more aggressive than others and this may not work with yours), harlequin and brilliant rasboras (never any problems there), a dwarf guorami (had to introduce them to eachother very carefully...started with a tank divider for a month and then rearranged the tank decor just before removing it, but now they get along fine), mollies, platys, danios, and snails. If you want some tank mates for him the rasboras (a small school of 7 or so) would work nicely. They are a nice, quick contrast to the slower moving betta and they don't fight or reproduce constantly. He won't pick on them either. One larger "Mystery" type snail would be fun, too, and would help clean the tank.