@Beardie03 your concerns are justified and I am going to explain why and why I disagree with the majority of what has been said so far.
By the way, I am vegan.
A person can absolutely be nutritionally balanced on a VEGan diet, and this is what is missing from your son's diet and the majority of the recommendation you have received here.
To be nutritionally complete, covering all your minerals and vitamins you need to eat whole food, whole grains, dark leafy greens, legumes such as chickpeas and lentils, tofu or tempeh, seeds colourful vegetables and fruits.
To make it short, what matters is what you eat not what you avoid, in this case, animal products.
In the case of a well designed vegan diet, the only supplement you need is B12, but in your case you need to supplement everything because I don't see the calcium, the magnesium , the zinc, the vitamin, the iodine, the omega 3 (EPA_DHA very important for the brain)
Oat milk is very sweet and the most milk-like product but also the most nutrient void compared to the high protein of soy or the calcium of almond milk.
All the ultra-processed food such as the fake sausage, fake burgers, fake whatever, protein shakes made with extract should be limited to rare occasion and not the staple of the diet .
Ultra-processed food is junk, it doesn't matter if vegan or not.
A human will build his bone density capital during adolescence so calcium rich food are very important, kale is high in calcium, broccoli and despite what many think, spinach is very bad because its calcium can't be absorbed, so calcium on paper but not on the body.
You should also remember that calcium is needed for teeth as well. Calcium needs K2 to be absorbed in the bones and not in the arteries. K2 is present in meat/eggs and the vegan sources are fermented food, so sauerkraut , natto.
It is true that protein minimal need is 0.75g/kg but this isn't the optimal need which is probably higher in a vegan adolescent.
You say he has done his research however I wonder where because ion the same way you the mum came to Mumsnet, a teen will search onto on social media, tiktok or YouTube,
You need to buy and both of you read a vegan nutrition book to understand the very basics of vegan nutrition . www.amazon.co.uk/Becoming-Vegan-Comprehensive-Reference-Plant-based/dp/1570672970/ref=sr_1_1?dchild=1&keywords=becoming+vegan&tag=mumsnetforu03-21&qid=1623383838&sr=8-1 this is a good one.
A good nutritional analysis tool is cronometer.com. Free of charge and way superior to MFP. It will highlight the nutrition content of the food you log. The line turns green, you are good for that nutrient.
Of course, the nutrient on a database for an apple might be very different from the apple you buy and most importantly , the body 's ability to absorb and use a nutrient is determined by many things including genetics.
Even with the most carefully designed diet plan, revised by dietician , one can very easily become malnourished . You should have blood in 6 months time to see where you stand. Including the Omega 3 index.
I strongly recommend to take a quality multivitamin while you figure out how to get the nutrients from the diet.
Being vegan without the VEG part is a path for malnutrition and probably weight gain . You should verify the origin of his "research" in a non inquisitive way, say you are interested in knowing more, and t tell you about it.
So far, the diet you described he is eating is like a dessert with the exception of his marmite sandwich. Try to switch from the sweet and processed breakfast porridge to rolled oats, with flaxseeds (grounded) , some blueberries. More fresh fruit than dried apricots (to prevent mould they add additives, read the packet).
It is a learning curve. Will his diet be perfect , no , especially not the packed lunch, but you control dinner and breakfast and weekend, so it is what you buy and cook that will make the difference.
The whole family could have several meals which are accidentally vegan such as soups and Dahl, Mexican dishes with rice, black beans , avocado, tomatoes, onions...
It might actually be a great thing for the whole family. Involve your son in creating a dinner with 4 vegetables 1 grain, 1 legume.
Have cooked quinoa in the fridge and he can add a spoonful to his plate. Think roasted veggies in the oven and you add marinated tofu slices for him.