They don't need a lot of protein, and it doesn't have to be in the form of meat.
Nuts/seeds (you could start with almond butter or sunflower seed butter if you don't want to try peanut butter yet) are good.
Beans are good. (Leave the chicken out and see if he'll eat them. My daughter prefers them whole, not mashed. She LOVED hummus when she was your LO's age.)
Lentils are good.
Cheese/yogurt/milk is good if he can tolerate dairy.
And whole grains are good too - brown rice, oatmeal, quinoa.
And I found that just giving her the grilled chicken we were having, cut up, and ketchup to dip it in was all she needed.