From a formerly intensely emotional child who became a very easy going teenager: don't abandon hope.
For me I didn't learn to control my emotions as much as I learned to control my perspective and filters. Which I did By building up thoughts and questions to make myself understand my reactions to situations and rethinking my actual sense of value on those.
You might be able to appeal to his curiousity about what triggers his thoughts to lead to his strong emotions. Maybe he'd enjoy reading about cognitive behavior therapy.
Kinda guesswork, because I'm not sure if I could've been led to that place by anyone but myself.
