I mean, I have been requested to add to this but i am not a nutritionist. *BUT* I am an ectomorph and have difficulty gaining weight as well. It basically comes down to calories-in, calories-out. It sounds like he’s got a high metabolism given his age (assuming he’s young given you look young). There are lots of high sugar option out there but the big thing is you want good calories. Get him to start eating fruit throughout the day. That will kickstart his appetite. Usually within an hour of eating something nutritious but also high in better sugars his stomach will begin asking for more. Carbs like rice, potatoes and sweet potatoes can be done up in very tasty ways.

These are what have worked for me but, again, i am not a nutritionist. Try it but it's definitely more than just protein shakes. He needs to eat more volume.