Being a professional author, I have found this to be a very common problem. This is one of those cases of: what the writer means vs how the reader interprets it. If we are to interpret these two sentences in the most-literal sense—regardless of what you mean; and offering, as they do, no other information but what might come before these sentences—“going back FOR it” suggests going back for (some, or more) alcohol; whereas “going back TO it” suggests going back to alcoholism (going back to drinking). In other words, going back FOR A bottle vs going back TO THE bottle. See what I mean?
So, if “it” is referring to alcohol (the drink) as apposed to alcoholism (the addiction), then “He keeps going back for it” is the more-correct sentence.
That said, however, neither of these sentences is a complete thought (or complete sentence), which causes this problem in the first place. The easiest solution is to simply offer more information—“Because he can’t ‘just say no,’ he keeps going back for it.”
Hope this helps.
NOTICE: The content of this comment is based on and according to American Standard English (formally American Broadcast English), the most correct form of English in the world today.