Answer: There are four main areas that could be causing his problem: phonics, reading comprehension, vocabulary, and fluency.
A reading diagnosis needs to be made to determine: a) whether he can sound out or decode the words at the level at which the text is; b) whether he understands the vocabulary words at that level; c) whether he comprehends what he reads; or d) whether he can read fast enough to complete the test in the given time.
I suggest that you select a book at his grade level and listen to him read to you. If he is struggling with many words, then a more thorough reading diagnosis needs to be made to determine which of the above four reasons are causing the problem. Once that is determined, then he can get help in the appropriate area.
Keys to Reading Success can help you do all of this at home with its easy-to-use, online reading diagnostic test.