while animal meat is the easiest source of a complete protein you can get complete proteins by pairing several sources of protein rich foods. In fact many traditional foods are nutritionally complete. You know beans greens and corn bread? you have iron, complete protein, fiber, complex and simple carbohydrates, which are important for every meal.
If you look at most cultural food pairings that have little meat they usually form complete proteins. It is interesting how traditional food pairings in different cultures ended up forming the nutrients necessary to give us the protein we need. Remember having access to a lot of meat is a pretty recent thing in human history.
It won't be easy for him to adjust but if he does his research and is careful, maybe use different cultural food pairings (beans and rice, beans and grains, beans and greens, or most any legumes instead of just beans) as a guide he should be doing pretty good. You can step it up a notch by researching traditional foods of your family's cultural back ground
Still research research research. If done right it is extraordinarily healthy, if not done right, it can lead to a lot of health problems, but so can a meat rich diet. If all you eat is bacon and fried fish you're not doing yourself any favors either.
You may also want to see if he wants to eat animal based foods like milk cheese and eggs that aren't meat but do have protein in them. IN fact you may be able to get him involved by having him grow a small garden. This way, he can be more involved in his meals. Many garden plants are very easy to grow, can be grown in pots,a re easy to care for, and reward the grower with tasty produce for the table.
Good luck! Sounds like you have a pretty awesome kiddo on your hands.
