peas, corn, a little bit of lettuce, they also like cooked eggs and oats and oatmeal. Make sure to provide him with grit when he eats anything other then chick starter. Also be sure to not be feeding him to many treats - he needs to eat mostly chick food so he can grow like he should.
You'll want to limit the treats to no more than 10% of your chickens diet so they get the most nutritional benefit from their feed. This list is very helpful knowing what is safe (and what isn't) for chickens: