Leftover, my son has been missing, lost in his addiction for well over 10 years now, I stopped counting. But I know that if I heard from him today I would do exactly what you did, reassure him that he is loved and suggest he call the people who can really help and reach out...meetings, Salvation Army Rehab, detox (there is no medical cost in Canada).
Your story sounds much like mine, years and years of rehabs and letting him come home only for it to turn out badly every time, enabling (yes, I enabled for far too long, mostly with good intentions but enabling none the less). In the end I was the one who got so sick that I lost sight of my life and how bad things were.
I have been in recovery for probably 15 years now, went to meetings for over 10 years and I come here and practice my 12 step program every day.
What works best is that I pray every morning and give the care of my son to God, then I live in faith the rest of the day that God can do for him what I cannot.
Life is short, please live yours well, regardless of how your son lives his. You didn't cause it, can't control it and can't cure it. He will when he is ready and not a moment before.
Hugs from my heart to yours.