L.M.
Easy protein sources:
Turkey meatballs (Whole Foods has a brand in their frozen meat section that are pre-cooked; one just needs to microwave them, pan-sear them, or throw them in pasta sauce while heating it up) - my baby, husband, friend's 4-year-old and I love them!
String cheese -- I've seen/eaten both mozzarella and cheddar kinds. A stick of string cheese with say, an apple, is a great snack.
Rolled up deli meats with/without a slice of cheese in the middle. Just roll up a piece of roast beef or turkey, or whatever, with some cheese in the middle, and eat it like a burrito without the tortilla.
Or, you could use cream cheese instead of deli cheese. It would make the "roll" stick together better.
Peanut butter on apple slices. Or, on celery. Or, carrots.
Hummus with veggie sticks. Or, veggie wrap pinwheels using hummus. Or, cream cheese. Pinwheels are rolled-up large tortillas with meat/cheese/veggies inside. Once you've rolled up the tortilla, cut it into one-inch-thick pieces. When you lay a piece flat, it looks a bit like a "pinwheel." Make these ahead of time, and keep them in the fridge for snacking.
If he'll go for it, you could also offer a yogurt with a handful of walnuts or almonds in it. The yogurt by itself will metabolize too fast to be a long-lasting snack, but adding the nuts make it a better protein-y snack.
Good luck, and good eating!