I would start by taking the most serious mistakes to the boss. It sounds like it could be a lot of work to find and correct them all.
If Boss just tells you to deal with it, I would start with an email. Basically, I would give a couple examples of what he did wrong and tell him he needs to go through his stuff and fix all of the mistakes, and get the revisions to you by a deadline. And I would say when I would be available to help/give advice. I do think the guy should be thanked for what he got right, but the mistakes sound to me like things that should not be soft-pedaled. I would probably go with straightforward--you did X wrong, please fix it by Y date, ask me if you have questions.
Misspellings and context errors are just sloppy; but he should be able to fix them fairly quickly and easily. You may have to give him more guidance about data interpretation, but my goal would be to not walk him through every single case, but rather point out a pattern that he can follow on his own. My optimistic assumption would be that he wants to improve and do well, so this is his chance to learn from his mistakes. And, *I* don't want to have to mark every single thing he did wrong--after a few examples he should be able to spot the mistakes and fix them in documents you haven't even looked at yet.
I've been on both sides of the fence. I proofread a lot of things written by non-native English speakers, so if they're making repeated mistakes, I'll mark a few then give them a broader rule to follow: "Check your verb conjugations" or "X-Y or XY--pick one and be consistent" or "Use more articles (a, an, the)." I don't want to mark every single thing and then have them fix it blindly, and make the same mistakes next time--I want them to learn and improve, and make fewer mistakes next time.
Plus, I know that when I make a repeated mistake, or a stylistic choice my boss doesn't like, I'm embarrassed and/or frustrated when she keeps pointing it out. I'm embarrassed enough that I spelled something (a name, say) wrong, and I will immediately go back and correct it ruthlessly--mentioning/marking all dozen times I spelled it wrong just compounds my embarrassment. Or, "I want the section headings underlined." Okay, noted, I will go back and do that. Making a big deal out of how the section heading isn't underlined every single time, when I haven't had a chance to fix it yet because we're still here talking, just makes me frustrated.