Here is the website for Idaho
http://www.healthandwelfare.idaho.gov/DesktopModules/Articles/ArticlesView.aspx?TabID=0&Alias=Rainbow&Lang=en-US&ItemID=1108&mid=10281
Follow the request to modify support. Simply put the major change in circumstances is the change in residency/custody.
If however there is a court order stating custody is with mom, then you will have to file a motion to modify custody with the courts. Again you can do this yourself if it is agreed to (but I would caution you of the errors that can occur)
Idaho does provide pro se forms.
http://www.courtselfhelp.idaho.gov/forms_new.asp?CAT_ID=30