What ever he was eating when you bought him should be mixed with any new pellet so he can change over gradually.
Get the freshes pellet you can find. Not pet shop pellets, most are full of treats and have no expirery date.
If you can't get any of the better chinchilla pellets, like Mazuri, Tradition and the like, go with the best rabbit pellet available. Check with the rabbit breeders, they ussualy use the best.
Get a bail of timothy hay from a horse ranch in your area. Not cow hay it's fermented.
You can also give alafalfa hay in cubes or loose, but timothy and a good pellet is the most important.
Most everything else is a treat and should be kept to a min.
Give loads of chew toys, from popsicle sticks to apple branches, small animal chew blocks and cuttle bones are just a few.
Make houses and shelves from kiln dried pine and he'll have loads to chew and keep him busy.
He'll let you know what he likes and doesn't.