It doesn't help that his dad is very lean and can not put on weight himself. Its as if their metabolism runs high/hot all the time!
Boy, 7 year olds can be very picky eaters. Not knowing the parents dietary habits makes this a hard question.
Breakfast is huge. The medication should not have kicked in at this time. This should be a meal where he really bulks up. Many families skip breakfast - they cannot allow this to happen. If he is eating school lunches, this could also be a problem. He needs his own lunch and what he wants. When he gets home from school, a snack or a very nutritious drink (lots of chocolate drinks out there and high energy bars). Dinner is probably the hardest, but if the other two meals have been good thats ok. It will take some experimenting and time - but its time well spent.