OP I didn't want to read and run.
You are doing all the right things now, so try and keep calm until you can talk it through with your GP.
It might be a might be a medical thing, it might be an emotional thing, or it might just be some weird anomaly that corrects itself. Step away from DR GOOGLE!
Does he have friends? Does he have an online life...I would be making checks there.
Look at what you all eat-
Whole foods- no low fat, no synthetics
Good proteins- chicken, turkey, fish, quorn, steak,
Legumes/Pules- chickpeas, lentils, rice, cous cous
Veg/Fruit
Dairy-real butter, full fate yoghurt, cream etc
Carbs- pasta, rice, potatoes,
Things to add good calories-
Breakfast,
would he eat a super smoothie with toast/cereal/crumpet/porridge, would he have eggs?
scrambled eggs are fast and delicious. 3 eggs scrambled piled on top a toasted muffin with butter and a smoothie would be a good start to the day. You make the smoothie the night before and keep in the fridge.
Smoothies we like-
Almond milk, chocolate soy milk, banana, ice, almond butter
Mango, berries, banana, almond milk, ice and apple juice
Lunch-
Soup
sandwiches, whole wheat seeded bread- tuna mayo with avocado, chicken with cream cheese, pesto and tomatoes
sub rolls with various fillings
pasta with various toppings
After school snack the French call it Goute with I love
Cereal bar- make your own or buy?
Brioche bun and a piece of fruit
Crudités with dip
Couple of biscuits and a hot chocolate/decaff tea/fruit tea/glass of pressed juice.
Handful of nuts and a banana
Dinner-
home made turkey burgers with cheese, guacamole, rice and stir fry broccoli
stir fry veg with salmon and noodles
chorizo stew with veg and butter beans served with granary bread
lasagne
shephered pie
chilli and rice
lemon chicken with rice and soy beans
chicken/steak or prawn fajitas with guacamole, rice, beans and cheese
Pesto pasta with chicken, feta and olives
Fruit/yoghurt for pudding and sometimes a proper pudding
Sponges, flap jacks, brownies, ice cream,
Light supper-
Cheese on toast, crackers with various toppings, smoothie.