Good question, and I'd like to know the answer too.
My son is 4 years old, and just like your boy, it's not easy to get him to eat meat unless it's hot dogs or stuffed inside a can of raviolli.
I give my boy a drink of my protein shake whenever I have one, so I probably give him an extra 6-8 grams of protein a day. I'm not too worried about the protein because it is derived from milk. And, I'd give him more, but I have wondered if all the protein or all the additional minerals would be hard on his little system.
I've also got a daughter who will occasionally take a sip, but since she's only 1, this is a pretty rare exception. Again, I've worried more about the minerals than the protein.
It'll be interesting to hear what others say.