Well, my stepson is 4 and was born here, and he is a very healthy eater (he will actually pull a whole cucumber or tomato from the fridge and just start muching, so he takes his veg to the extreme... haha). So I don't worry too much about his vitamins, since he mostly just eats fruits and veg, but he doesn't like many protein foods and he still doesn't like my cooking very much, although he enjoys helping me. One thing he loves is to pop popcorn with me. I just put a bit of oil in the pan, he stands on a chair and pours some kernels in, and then he gets to put the lid on and watch it pop. I just put a little bit of salt on it and he eats a huge bowl by himself.
Other than that, he will eat spaghetti bolognese (though he likes marinara better or cheese ravioli from the tin), fried eggs on toast, fried rice (I once made pork fried rice and cut all the meat into really small bits and he loved it), fish cakes (sometimes he eats my homemade ones but he prefers the frozen ones, to my dismay), hot dogs from the tin, Birds Eye chicken dippers and fish fingers (he didn't eat my homemade chicken nuggets), meatballs from the tin (again, he didn't like my homemade ones - are you sensing a theme here?), and he likes grilled cheese sandwiches with ketchup. He will also eat most of the veg on a Sunday dinner and the Yorkshire puddings, but not the meat or potatoes.
Have you tried him on grilled cheese sandwiches or cheese quesadillas? I don't know what to do about the vegetables and fruit, but for my stepson, I usually just don't allow him to eat any junk food within two hours of his meal so that he is hungry enough to eat it, and if he asks for something in particular (yoghurt or ice lolly or whatever) then I tell him he can have it after he eats some of his dinner. It seems to work ok for us. We do spoil him a bit since we only have him every other weekend and let him just graze through the day sometimes, since he will mix in fruits and vegetables with his junk food on his own. Maybe you could try chocolate covered strawberries or things like that to see if he will try the new foods, then he might find he likes them.