I've seen my son relapse when his life was wonderful and on track. I've seen him stay sober through some grueling stuff.
It's not about relapses, I think, it's about diligence of recovery. Many recover through many programs or means, but something I hear as common ground from recovering addicts is that they have to focus every day on their recovery.
Complacency is a relapse waiting to happen, according to my recovering friends. I know that is true of my codependency.
Hope your son finds recovery again, he's had a taste of sober life and he may just want that back again.
Hugs