Talk to his English teacher. Take the time to talk and tutor him along with positive encouragement! My son had this issue and as a parent I felt responsible to help him correct it.

Hope he's not on FML!

Maybe he was hoping you could give him some feedback and constructive criticism. You should also help him fix the grammatical errors.

I see that you're trying to be nice, but it wouldn't hurt anyone to get constructive criticism. At that point you'll probably feel like throwing the thing down and yelling "This is terrible"; I've been down that road. Have him sit down with you and go through it together, pointing out the grammar mistakes and plotholes, if you think there are any. (:

You don't have to finish it. Skim. Then tell him that you appreciate his efforts, your proud he stuck with something he's passionate about and that you support him completely but that his book needs a little help. Don't bash everything about the book. Do baby steps like ask him questions about the plot line. Now go to the book store and buy him a few books on how to write a novel and improving your vocabulary and what not. Encourage don't discourage. Good luck.

What is it about?

Completing a 300 page manuscript-even a horrible barely edited first draft-is quite the accomplishment at any age. I'd encourage this interest by a visit to the book store to pick out some writer's guides, try to find ones that focus on grammar and story development, and how about a pocket friendly dictionary and a thesaurus-yes it's the digital age but they still come in handy. Also, possibly research writers groups or workshops in your area, they accept any age usually and it might be good from him to get some feedback from non friends or family.

at least hes trying he could hate you and want nothing to do with you be happy hes leting you read it

Lol!! :-)

The best way to offer criticism is to "sandwich"it between to positives. For example, "I'm really impressed that you're able to dedicate yourself to things that are important to you, if you like I can proof read and fix up any grammatical errors you have made to save you time. I really like how you've developed ___ part of the story. I'm guessing if he is asking you to read it, it means he values your opinion and trusts you not to ridicule his attempts.