You need to explain to him that his body is developing and he needs to feed it to help it develop into a strong, healthy body for him to have for his lifetime. Whereas, (like me) our bodies are not "growing" - they have finished, so Mommy and Daddy have to be more careful of what they eat so they can keep their bodies healthy for their lifetime.
Kids pick up sooooo much negative body image from TV, peers, everywhere. He may have heard you and Hubby talking about "being fat" or loosing weight, and he may want to be like Daddy - so you have to really teach him the difference between growing bodies and adult bodies.
Encourage him to eat more fruits and veggies - they are healthy, but not "fattening" foods. I limit the carbs we eat anyway, and have always told my son he can eat unlimited fruits and veggies.
My son (15ish) is officially underweight according to his last checkup. So, we have been having lots of discussions about weight gain. He told me last night, he would rather be underweight because it looks better. Ugh. So I explained, once again, that he needs to at least begin to build muscle tone - muscle weight, not fat weight, and be healthier. While, I, on the other hand, need to shed about 15 pounds. Balance our meals is becoming a challenge, as I am trying to encourage him to eat more, while cutting back on what I eat.
Hopefully, this is just a phase that your son is going through. Communication is key here, serving him a variety of healthy, nutritious foods, and teaching him about the food pyramid, and healthy eating choices.
My son, will most likely always be "officially' underweight, as he is his father's clone, but, teaching him to eat healthy, and be healthy, will serve him well for life.
Good Luck
God Bless