How well is your son comprehending what he reads? He may be very capable of reading the words in Wimpy Kid but not fully grasping the meaning of what he's reading. Reading level includes both the ability to read the words and the ability to comprehend what you're reading. If he's not reading quickly, he may be struggling to understand everything he's reading, and therefore takes longer to read it so that he can try harder to get the meaning.
Talk to the reading teacher directly to get more specifics. Set up a meeting, or at least a phone call, as things aren't always communicated well via email. Most importantly, ask what you can do to help your child catch up.
He should be reading out lout to you for about 15-20 minutes every day. After he reads, ask him questions about the text and see how well he can answer. Have him come up with some questions to ask you as well, as that will further gauge his understanding and force him to think about what he has read.
Finally, while I don't think there is anything wrong at all with the Wimpy Kid books, I don't think they are a great way to judge reading ability. They skip around more than a regular (non-diary style) novel and can be a little harder to follow. Have your child try reading things like the Mouse and The Motorcycle series by Beverly Cleary, or the Tales of a Fourth Grade Nothing/Fudge series by Judy Blume.