@arishin In addition to therapy, something you can do to help him is to give him real responsibilities within his abilities and interests.
Perhaps he is capable of planning your next family vacation. You give him constraints and let him go. For example, our budget is $X, our days off are Y to Z, we can handle this many miles of driving to get to the destination, and we'd all really appreciate it if you try to do something each of us will like at least once. He can plan National Parks and camping or hotels and museums or an amusement park or anything he wants to (and perhaps can use fewer days if necessary to stay within constraints).
If that doesn't seem feasible, maybe he can plan the grocery shopping or maybe he can coordinate chores for the family. Giving him both authority and responsibility can make a big difference in his self-confidence.
Another thing you can do that may be more effective than a gratitude journal would be an observation journal with prompts you change. Maybe come up with 7 prompts for 7 weekdays with occasional additional prompts. Perhaps something like "notice something you can genuinely compliment", them talk about how that comes naturally for some people, but he may need to work at it. Maybe an additional prompt would be "earn a C or lower on a quiz at school", then talk about the kids who earn Cs all the time and that almost all of them will grow up into productive, good people, how grades are not always a good source of self-worth, but that it can be hard to recognize good sources of self-worth in a society like ours.
I give these suggestions because I had some similarities to him and I learned a lot of useful things on my own after too long because of my own reflections, but guided reflections would have really helped me, and because the most life-changing thing for me was a big trip that I planned at 16 and my parents trusted me...they didn't double-check, but were there to help when things outside my control went wrong and I was stranded (cancelled flight). I think I would have been a lot better off if I could have planned a trip five years earlier!