A foods, we add cooked quinoa and well cooked beans for the additional protein. You can also offer well cooked chicken or well cooked boned fish. Maybe once a week or so on the animal protein, and in small quantities.
Eggs are also an excellent source of protein.
You can offer well cooked scrambled eggs, or hard boiled eggs a couple of times a week. Since you have egg in his diet already, I wouldn't necessarily add more. Hope these ideas have been of help. Toby,four years old, rescued last year by me. Calcium deficient Clipped wings, stressed out with two dead wives and various owners.