I think it's wonderful that your son makes such a great decision at such a young age! It sounds like you're doing a great job in supporting him and being a parent.
I agree with what the others have said. Quorn et.c. are convenient but not that tasty. Beans and lentils are great to replace meat in some dishes, I personally like to let them replace most of my rice/potatoes/pasta/other carb as well. Nuts and seeds are great as well.
Try new foods! Lentil stews with tomatoes or other vegetables as a base/sauce, lentils (obviously) and spices, serve with rice or other grain. Quinoa salads, with veggies, maybe some beans, fresh herbs and a drizzle of olive oil. Hummus with pita pockets and/or veggie sticks to dip. Chilies (you can just leave out the meat). All sorts of bean burgers, falafel, chickpea patties et.c. Soups. Nut butters (such as peanut butter, almond butter et.c. - make your own my mixing the nuts in a blender for several minutes to avoid scary additives) make great sandwich spreads. They can also be added to pasta sauces, soups or desserts.
Nutritionally, if he still eats dairy there isn't much to worry about. If he doesn't (has decided to be vegan) he can still get everything he needs, though it does take a tad more thought and planning. I suggest you read a bit about nutrition - a growing child is vulnerable to deficiencies, especially if he's a picky eater. Key nutrients are iron, zinc, vitamin B12, vitamin D (is a common deficiency among omnivores too) and calcium. This website
has great resources!
I wish you the best of luck!