Dairy is an excellent source of protein, especially cottage cheese and yogurt. Peanut butter is also good. Make sure he's eating some foods that contain iron, also, since meat is where most people get their iron. Most breakfast cereals are fortified with iron. It's really common for toddlers to go off meat or refuse it altogether (neither of my kids have ever eaten it), and it's still possible for them to eat a balanced and healthy diet without it.