There you go then. Don't worry about protein powder. Just say, milk isn't expensive so drink up. If he doesn't like milk, eat another animal protein source that he does like. If he wants some milo with milk, go with that.
Only a few years back, My grandma took my great grandma to the Doc and said she eats too much salt and sugar what should she eat. The Doc said who cares, she is nearly 100 stick with what works. The oldies usually just need to eat something, they often don't eat enough full stop.