I am no expert. But this worked for me...
They had me eating lots when trying to put on weight. Big breakfast, 2-3 school lunches, footlong or a big burger after school, dinner and more dinner before bed. I swear i was eating 6000+ calories a day, probably more. It worked because of how active i was with wieghts and cardio + practice. I was able to add mass but not a ton of fat. I went from a 6'2", 155 lb freshman to a 6'3", 195 lb senior and still had very little body fat. I was just feeding my muscles. However the problem for me came when i was done with HS and a few years of college. As I stopped being as active I still kept those eating habits.....not so good. LOL
I think adding Whey protien to his diet wont do any harm, but you need to increase his fat intake too. As little as he is he is probably burning energy by just sitting at the table. The extra fat and calories with feed his body and growth. The protien will repair his muscles and can help with some bulk. I would just be careful about how you change his diet. After 4 years of constant intake its hard to turn off.
But as you know HFIOL...every kid is different. He might just be a rail.

best of luck to him and keep us up to date on his progess through the season.