It could be anything from him figuring out that you give him attention for limping to a microfracture. My son had microfractures (they occur around the growth plates)from a fall and these were undetectable on an xray. We ended up in the ER because he couldn't walk and they couldn't tell us why. If it is microfractures they generally heal quickly. My son was back walking w/in 2 weeks and he's fine now (it happened over a yr ago).
I would definitely get him checked out. AND I would go to an orthopedic dr if necessary. My son's ped said he was fine, nothing wrong. She was VERY VERY wrong!
mum to 3, 8 yo dd: 6yo ds and 4yo dd