Try finger foods, anything he can hold in his hand. Rice cakes, they come in many flavors and dissolve in the mouth easily. Cheese slices, banana, cherios, crackers, graham crackers, veggies (peas and carrots, green beans, corn, canned or frozen), cut any meat into little pieces, just put on the tray and let him play. Let him make a mess on the tray it will wash up, he'll wash up. Try apple sauce and yogourt, let him try the spoon. Spagetti O's are another thing he can try with a spoon. In the summer give him pop cycle and let him get sticky outside. In other words let him try to feed himself. Give a vitamin if you get worried about that, as long as the doctor said he is gaining enough weight he'll be fine.
After the first year their growth slows a lot, so he his not going to eat like he use to, if he is not interested then don't force it, as my dad use to say "he'll eat when he hungery.