Are you using the same brand and variety of pellets that he was eating before you got him? That’s assuming he was eating pellets before. Rabbits can be picky about food and can shun something if it’s different than they are used to. If you don’t know the brand he had before, you should call and ask, and ask about his prior eating habits too. If you are using the same brand, then I’m not sure why he would not want them now – unless you happened to get a bag that is not fresh or has something wrong with it. It is good that he is eating his hay, but young rabbits like him should also be eating pellets.
Whenever a rabbit isn’t eating as he should be, there is a possibility that they have a problem with their teeth. Since he is so young and is eating hay, his teeth are probably okay. But it’s always a good idea to take a new bunny in for a vet checkup, so you may want to go ahead and schedule that.