My 13 yr old son is 5' and 69 lbs. Docs not worried about his weight. What should I do?

I'm worried about my son's weight. We've tried high calorie diets, protein and weight gain powders, high carb foods and he still won't gain any weight. Blood work doesn't indicate any issues with his pancreas. His BMI was 13.9 and he's below the 0% on the growth charts-he hasn't gained any weight in the past two years. Am I worried about nothing? Any suggestions would be appreciated.