As someone who was raised vegetarian, I would say that meat really isn't that essential. The truth is that most Americans get far too much protein as it is. You might like to read the book "New Diet for America" to learn more about this. There was once a lion that refused to eat anything with blood in it, making her a vegetarian. You can read the story here: http://www.vegetarismus.ch/vegepet/tyke.htm
Here's what you can do in the mean time. There are a lot of "veggie meats" that you could use to substitute. Mixing grains and legumes (beans and rice, for example, or beans and tortillas--think tacos, or peanut butter with bread, etc) creates a near complete protein. If he'll eat milk and eggs and cheese, then serve them. Tofu is high in protein, but I wouldn't use it every day (we eat it 2-3 times a week usually).
How do you cook tofu? There are a lot of good recipes on the Internet, but I would say just experiment. One easy way I like to do it (and fairly quick, too) is to dice it and season it with Chinese 5th Spice and a little ginger (you don't need much of either, just a few dashes per serving of the 5th Spice and 1 or 2 of the ginger), add enough soy sauce to where you don't need to add salt, and let it sit while you chop some onion, saute, and then add the tofu with chopped mushrooms and cook until the mushrooms are done. This goes well with a savory main dish. We often eat it with hashbrowns or tater tots. Try using cumin, chili powder, and salt to season the tofu, and cook it with onion, tomato, bell pepper, and cilantro. Or crumble it with basil and oregano into spaghetti sauce instead of meat. Or use it instead of ricotta cheese. Or slice it about 1/4 or 1/3 inch thick, bread it (use soy sauce or something salty to make the breading stick) and pan fry or bake it to use in sandwiches. You'll need to add some salt, since tofu has no salt on its own. Mashed with a chicken-like seasoning (think powdered chicken broth), you could add mayo with diced onion and celery and make "egg" salad sandwiches. Okay, so some of those ideas aren't exactly substitutes for meat, but it should give you a few ideas on how to use tofu.
And don't worry. I grew up eating cooked cereal 5 times a week with toast, and we didn't eat a lot of protein, and I turned out fine. My brother did too. There are plenty of options now, more than there were when I was a kid, so you shouldn't have to worry.