Are you able to perform a "restore to factory settings" when the iPad is connected to iTunes? It will erase all content from the iPad, including the locked issue. If this new PC has had the iPad synced even once before your son's attempts locked it, then "restore from backup". If not and you are however, syncing to MobileMe or iCloud you can pull down whatever the content you have up there after the factory restore and you set up the iPad as if it were a brand new one.
Good luck Gary, double check before you do a restore, what content you are likely to lose and email it yourself if you have to, in order to save it.