I decided to become vegetarian aged 7 - the rest of the family are all big meat eaters - and as an adult I am so grateful that they were respectful and considerate of my choice. I'm still veggie 30 odd years later. So, firstly just I think it's fantastic that you're supporting your son like this.
I gave my own kids meat as little ones, but the eldest two have since decided to go veggie (they were around 7/8 at the time).
In terms of nutrition, my advice would be to worry less about meals and more about their daily protein/ vitamin intake - so adding nutrients into breakfast, snacks, puddings etc.
So, when they get in friom school I'll give them a snack of something like hummus with carrot/ cucumber/ peppers to dip, apple slices dipped in peanut butter, banana and peanut butter on toast, oat crackers with hummus or peanut butter or dried fruit and raisins (dried apricots are great for Iron which you also have to watch).
For puddings, I'll give them Greek yogurt with fruit and crushed up nuts, or a home-made banana split with berries and crushed nuts on top.
For breakfast, porridge with fruit and nuts on top is good, or Weetabix - again with fruit and nuts.
It's pricey bit Flaxseed is great - you can add it into porridge or pasta sauces for a bit of extra protein.
Their fave meals are -
Veggie Chilli (I do it with loads of different types of bean rather than a meat subsitute) - they especially like it with tacos
Veggie Spag Bol (I use lentils instead of Quorn)
Veggie burgers with potato wedges and peas
Veggie Pasta Pesto - I cook peas and brocoli in with the pasta and then add cherry tomatos when I serve it.
Bagels with scrambled eggs and cherry tomatos
Tomato Pasta
Pasta Norma (this is like a tomato pasta but with aubergine) Pasta alla norma recipe | BBC Good Food
Roast veg and halloumi with pitta bread
Chickpea Curry