HI and welcome - I´m a mum to a 12 year old O as well ( a girl)
She has been living this way since she was 5/6 years old
- but it depends how strict she follows it -BUT never wheat or corn in her diet !
Loosing weight in kids:
goal is that his body weight stays more or less at the same level
- but that his body grows...
- it is far healthier than loosing a lot weight when you are a kid (small loss are OK)
At least that is the approach the docors use here sucessfully with growing kids.
Kids are not mini adults
(pity that it can´t be done that way with adults

)
Since he is 12 - I guess he is just about to grow really fast - so that should be no problem.
About protein ( Why not give him beef etc at lunchtime - when he doesn´t eat with you- and feed hime fish and turkey/chicken at dinner?)
Do he like eggs ?
My daugher has eggs in some way most mornings
- that way she can have her grain serving at lunch -but not all days
- the O´s should really only have grains occasionelly.
Protein in the the morning is the key for her to stay healthy and happy.
Good that he like his sport- so crucial for O´s.
To show you how she eat :
Monday- friday
BF softboiled egg or fried egg,sometimes homemade turkey patty- 1-2 pieces of fruit( always banana and some other benenficial fruit)
Lunch in school is either leftover meat and fish from last dinner- and some kind of veggies- she is getting hooked on romainsalad with chicken, tomatoes etc.
Some days she has ryebread or white spelt sandwich bread with cold turkey - fish and mayo- carrots and red peppersticks on the side. Sometimes she has small mozzerella cheese balls or creamy fetacheese.
Water to drink .
Afternoons depend on if she do sports
- she has more filling snack with more protein and veggies- or a smothies with hoemade rice/almondmilk banana berries - flaxseeds etc.
If she is just at home- she often just have a handfull of walnuts, pecans or almonds and 1 piece of fruit - maybe some herbaltea. Red fruitty teas with hibiscus - sweeten seem to be well liked by teens.
Dinner- 2-3 types of veggies and some protein
Remember good fats for growing kids.
I aim at 1 tbl olive oil and 1 tbl ghee or butter for her everyday.
In weekends she is allowed do it a bit different:
She has more bread/grains- mainly white spelt bread- cake or ricenoodles.
She can have some cheese as well- mainly mild goat or sheepchese
- somtimes she allowed to have some of my organic regular cow milk cheese
- but it is really not the bedst. She has small amouts of cream and sour cream in the weekend as well- the curse of living with a B
Sweets ?
Well we stick to homemade cakes - sorbets and dark chocolate- only weekends- and it seem to do the trick.
Sodas ?- homemade version- using mineral water( bobly) and fruit concentrate without avoids. Cherry or blueberries are a hit !
If she is invited to a friends house - she seem to regulate her food- sure she has some avoids- but she seem to be getting better and better at choosing more correct each time- I think the fact that she got so sick after a trip with school- made her aware on how she feels when she eats a lot of c**p. NOT worth it.
This friday she went to a girl party and came home and told me she hd done her best: avoided the popcorn and tacos- but did eat some hamburges with the bun - they had meat and tomatoes and salad- Did not have the sodas, icecrem and candies
- but did have the chocolatecake and some orange/pineapplejuice.
I think she did fine.
She still need to be social.
Good luck he sounds like a really wise kid