Stop feeding anything but the pellets for now. Before you run out and buy ten different kinds, you can try squashing up the ones you have a bit and try those. (If you feel they are too big) You can also presoak them in some tank water to soften them up a bit, then put them in the tank for him.
Offer the pellets, remove what goes uneaten. If he doesn't eat, try again the next day.
He won't starve himself and he should be eating pellets as a main source of food...everything else is just treats. Sometimes it's a contest of wills...a healthy betta can go a couple of weeks without eating. I don't think he'll wait that long.
Once he is eating the pellets regularly, you can reintroduce the snacks.
Mine took almost a week before he would touch the pellets. He was used to being fed dried bloodworms and refused the pellets day after day. Now he constantly begs..lol.