So sorry to hear about the setback. It seems that recovery is rarely a staight line.
It sounds like your son has the skills and determination to get back on track.
Wishing you well. I tell myself that when my son relapses it is my job not to relapse with him but to keep working the skills I learned via Al-Anon so that I
keep my own sense of equilibrium and gratitude fpr the life I have.