Don't worry. My son has similar problems and it has been going on for a long long time.
I also worried a lot about him not having enough nutrition, but since he seems to eat snacks, I try to put some meat/veggies in his snacks to try to make him eat more.
Now I'd let him eat things like:
1) Meatballs (with fish/grated carrots/pork/flour): bake in oven
2) Pancakes with cheese/fruit jam/raisins
3) Potato patties and add some meat/veggies: bake in oven
etc etc.