I think your best bet would be to start looking for an entire replacement unit on eBay or in your local wrecking yard. There used to be a guy on here that i thought rebuilt them but i don't remember. As i recall it was a little plastic wheel internally that would break causing the odometer to go out, sounds like yours is both though so i'll let some of the more seasoned guys help out. Me personally am going to a custom instrument cluster in order to avoid all that.
where and how much did your son's new speedometer cost. jj