This is a great place to start, all you can do is get the word out in as many places as you can with as much information as you have. I'm definitely not an expert and am very new on this site but I have to say the people on here are very helpful and knowledgeable. As a Judge owner I verified mine was an actual Judge through PHS, they provided information to me such as build date, and where it was originally sold. So posting the year, color, interior, transmission type, and where & when it was purchased would be a great starting point. As his first car it probably wasn't new off the showroom floor so that might have some added challenges.
So to knock one off the list, if he purchased a 69 Matador Red four speed with black interior from Casey Beckham Pontiac in Anaheim in Feb of 1969 it could be mine. It some how made it's way through Texas and then up to Canada, it's been grazing in Saskatchewan Canada since I bought it in 1979.