With 20/20 hindsight, your DH& DS at a open air music festival camping overnight(s) & considerable drive from home was over optimistic for both of them.
Even if your DH had never had an issue with alcohol or touched a drop, the overall environment - noise, open air, loads of unknown people, & change of routine & surroundings may well have proved challenging for your DS. But had your DH been present, then any distress of DS’s would most likely have been minimised, but they might have come early.
The same disruptive factors would also have an effect your DH & prolly undermine his sobriety. It has to be said that a music festival is not known for temperance & so I assume that your DH has actually been hanging out of the wagon for a wee while. Were he serious & truly committed to sobriety he, & his Sponsor/peer group would have called this off before the big adventure started.
I support the suggestion of you going to Al-Anon. I think it will be a more bracing & direct support group for you, & also for your DS.
I accept that you thought he was doing very well, but alcoholics lie. They lie to themselves & everyone around them, & they become more adept & daring in their deceits as the condition takes a greater hold. How he could abandon your son at the festival is unforgivable, but his justification that drinking would be not be a problem & your son would be OK will be built upon lies he is telling himself.