Help in reading and math?

If he is mentally challenged, then he should be referred to professionals who are trained to assist students like him. This sounds much more serious than a tutor or friend should be strapped to do. I applaud you for helping, but it doesn't sound as if you are equipped to give him the help he needs. One thing you can do is drop down a level and see if he can absorb what is expected of him from the 4th grade level. If that works, then you might be able to ease him into the fifth grade level. This is one of the reasons why I am not a fan of NCLB.

Well unfortunately, you haven't really gone into detail with what he is struggling with. Is he able to sound out words? Is he not comprehending what he is reading? I would suggest that you choose a passage to read with him. Write down any words that he is having trouble with. Review those words with him. Talk about phonics rules etc. and then have him reread the passage until he knows it. Unfortunately, there is not a magic way of making him a great reader. It just takes time. Some children develop slower than other children their age. Like the old saying goes...Practice makes perfect.... If he is not comprehending, then you need to stop him after each page and ask him questions about what he read. I used to be a reading interventionist at a school and I used a program called Corrective Reading. It's published by McGraw-Hill. You may be able to find a copy of that online through amazon or ebay. It's a pretty good program.