His relapse started long before the day he drank beer and smoked pot. They don't just wake up one day in active recovery and say I am cured - they stop actively recovering first. Then their Alcoholic/addict voice starts whispering to them. The voice grows stronger as they pull further and further away from recovering. They stop going to meetings so much, don't hang with their AA buddies, stop talking to their sponsor.
His sponsor didn't ditch him - he ditched his sponsor. The sponsor spotted the tale tell signs long before you had a clue.
Does he believe he is cured? Who knows. He probably did before he used again. Now - he doesn't care what you call it. His old friend is back ......and he couldn't be happier.
This will progress. It will get worse and sorry, there isn't a damn thing you can do to stop him.
You can help yourself though. Get to Al ANON - You need it desperately.
Sorry for what you are going through - been there done that.