If he's doing an Independent exam in Jan 2012 then you have little time. If he's doing the exam at the end of 2012 then you have more time. I am fortunate enough to have two children who got through and comprehension is one of the most difficult areas.
Don't worry everyone finds it difficult, even adults struggle to cope with a whole page of deconstructed dickens.
Unless you are going for outside help ( not always good with comprehension anyway) I'd ( shock horror) suggest practice papers are NOT the way forward..... read lots, and I mean lots. Read classic books, do some reading together, discuss the story to check comprehension ( can be done in a fun way), play some vocab games like scrabble, get a list of synonyms and opposites from the internet ( a long list)..... it's really very difficult for a child to just leap straight into practice papers if the problem is with their core English skills. Doing paper after paper won't unfortunately boost their vocab for instance.
It's a really tough thing to do so the very best of luck. If you only until Jan it will be difficult to make much difference. One thing I wouldn't do is to do tests that are not the length he will experience in the actual test. The reason for this is that often the comprehension passage is quite and requires them to really concentrate for say 50 minutes. Better to get used to that than do lots of ten minute tests I'd say.
Good luck..... whatever happens the work you do will help so it's not going to be wasted even if they don't get in.
If you have more time then try LessonsinthePost if you want to. Worked really well for me. or perhaps a Private Tutor who specialises in 11 Plus (watch out for the cost) but word of warning it does require work but then again everything does if you want to compete. I wish others didn't do so much with their kids then we could just turn up on the day and we'd see if they were good enough or not.