Hi Jonathan!
Ideally, if the child was diagnosed early on with CP, he would have started hip surveillance every 6 months until the age of 2 years and then once a year after that. Since he is older and has not yet received hip x-rays, it is recommended that he now get x-rays completed every 6 months for 24 months to get a baseline and then once a year after that. So, in this case, the 24 month period starts now at the age of 4 years and then after 24 months (at the age of 6 years) he will start getting x-rays one time per month.
Here is a link to a nice visual depiction of this schedule: https://www.aacpdm.org/UserFiles/file/hip-surveillance-care-pathway.pdf