Welcome to the family. You will get a lot of support on this site and also on the Friends and Family side
Friends and Family of Alcoholics - SoberRecovery : Alcoholism Drug Addiction Help and Information
I think a good starting point could be to stop buying him alcohol which is basically enabling
-if I tell him no to purchasing alcohol he has huge tantrums until I give in. I hate it! I don't want to buy it!. But how do I say no? He'll just take my card and get it. Even if I figure out how to hide the card, I don't want to deal with his tantrums.
He will throw a few tantrums at the beginning but if you stick to your guns, sooner or later it will dwell on him that you will not buy alcohol for him and he will stop asking.
Maybe that could be your first boundary?
Boundaries are about you. You make them for your
own well being (in that case you hate buying him booze and it makes you feel bad) not for the addict. Those are
not threats, just facts.
Once you set up a boundary, it is also crucial to stick to it no matter what otherwise he will see it as a joke. Be firm and consistent and do not make it personal against him.
Just state calmly and firmly: I do not buy alcohol
Let him throw his tantrum and do not engage. He might take off and go get his own booze, fine...whatever. What is important is that you stuck with it.
Wishing you the best, I know firsthand how difficult it is.