You can pursue the seller via a civil suit, arguing that the terms of an (what I assume was) the oral contract between the seller and your son required the seller to deliver clear title to the bike.
I hope this was helpful to you. If it was please remember to click "ACCEPT" on your screen to make sure that I am paid for my efforts. By clicking ACCEPT you are not giving up your right to ask more questions on this issue, and I will be happy to respond to these as well.
Attorney
Practicing attorney for 17 years and principal of law firm.