May the peace of God which surpasses all understanding guard his heart and mind in Christ Jesus!
Forgive me, but if I may ... your son is blessed in a way as such a "thorn" draws one closer to God, puts things in perspective, and can produce much joy in things others take for granted. Where other young folks his age are absorbed with selfishness, he has a unique grip on the value of "now." While I pray for his healing, I pray also that his life has been enriched by his illness. I have been blessed to walk with thousands of young men roughly his age who've experienced life altering events that left them changed forever, and many have come out much much stronger as a result.
Again, forgive me if that is not how you or he see this, I do not mean to offend. Not everyone sees things as I.
Blessings, Chaps