I saw that one as it was in the area. I read that the "roof was resealed after we bought it".
And for 500 bucks I'd just buy it and tear into it as a project, assuming the frame, springs and axle are all good. I'd go see it, considering they only have the one picture posted.
My bet is your son will use it as a fishhouse or deer camp trailer. Long as it's sealed up, doesn't leak, not moldy...