On the other hand. If he is an alcoholic/pot addict, which to me he sounds like he isn't denying that at all, then to just stop cold turkey puts their bodies into withdrawals. Their ability to function in all aspects of life are pretty greatly diminished during this time. Just needing enough to keep them at bay "take the edge off", is his way of trying to keep on going with his work, life etc. until he can get into treatment.
He isn't asking you to let him keep on drinking/smoking. He knows he has a problem. He has tried multiple times, and openly wants to try again.
While normally I have little sympathy for the "excuses" and gas-lighting etc., sometimes I feel the need to remember, that this is a disease that they can't often get out of, without serious help. I don't think he wants to live like this. So he is trying to make the best of it he can under the current circumstances.