Is there a single word for someone who CAN drink a lot? He is not necessarily an alcoholic, but when he drinks, he can drink everyone under the table.
Most of the words that come to mind are archaic, which I don't think it what you're looking for. Interestingly, in Benjamin Franklin's time, you could have said someone was "biggy," which I could see coming back into use. Other things you could call your drinker would be soak or toper. I can't think of any modern word that doesn't also mean alcoholic, however. Here's Franklin's list of related words: https://founders.archives.gov/documents/Franklin/01-02-02-0029