How much? As much as he wants. I think he will forage around until he gets it. I sometimes wonder if some growing children eat tons of carbs because they are actually craving protein. You do get protein from grains but have to eat a lot.

I was surprised by your assessment of morningstar strips. I will have to check. I use Quorn stuff. the sausages are about 3, and the roast..Oh I can't remember now. But the real problem is that I find them a bit nauseating after a while. They are made of a kind of micro mushroom type thing, with egg protein. SO many veg versions of foods are made from grains. Some other sausages like 'cauldron food' are nicer. Cauldron foods make a very nice 'marinated tofu' which could pass as something else in a mince' substitute meal.
I eat fish, which helps. But in general I go for nuts, pine kernels, cheese and eggs to bump up the fat and flavour as well as protein.

The thing I notice about chubby children, funnily enough particularly boys (I am a teacher) is that they are SO inactive. Your friend may need to find a fun family activity for their son to be part of.
There is a very good thread somewhere about children, I will try to find it.