K.V.
My son has always been very slim. I was always of the mind that if they are thinking clearly, and have a lot of energy, then he is probably doing okay. (that was for my son). But, he isn't a picky eater. I do have a brother and nephew who are extremely picky.
I would always have a side dish that you are sure he will eat. Something healthy and full of calories and fat. And every other meal, make sure the main dish is something that he likes. I don't cook 2 things, but if I am cooking something, and one person doesn't liek sauce on the chicken (or whater), I take a portion out before I put the sauce on. Or, I leave the sauce off completely when possible and let everyone put their own on. Make sure that you have lots of snacks(preferable something somewhat healthy) available for him when he gets home from school. I have an open door policy on the kitchen, as long as they eat their dinner and other meals. (I might not do that if they were overweight, but that's not a problem you have).