I had to read this book in school for AP English. I didn't consider it overly socialist.
As well, might I make a point?
Let your kid read what he wants to read. And let the school assign him what they want.
Continue to stay the course as a good parent, and make sure that he can think for himself. Continuing to teach him how to see through their liberal indoctrination crap is the best thing you could give him. Knowing how to make positive decisions on his own will be invaluable. Sounds like you're on top of it, seeing as you actually care about what he's doing in school.
And besides. Even books with a 'liberal' message or tone may have something to teach. It's important that you teach him how to parse this in a manner effective for separating the useful bits from the trash.
Keep up the good work!