When you say he is picky, is he picky about all foods? Does he drink sodas and eat junk food? I had a niece who irritable had no energy who was allowed an enormous amount of sweets at home. I had a summer to wean her off that stuff (not without a fight) and she started to improve. It was a struggle because she seemed to be addicted to sugar.
If the food you serve is not the problem, then I would give him vitamin and mineral supplements. Because our soils are depleted, probably everyone should be on supplements. The B vitamins especially should perk him up.
His weight between the 25-50th percentile and his height is between the 90-95th percentile for his age. Suggest giving him a daily children's multi-vitamin due to his eating patterns. Suggest Boost, or milkshakes or smoothies with fruit {milk or yogurt, banana or strawberries (or any type fruit), ice cream blended} 1-2 times a day for additional calories for weight gain.