I have run into the same type thing a couple times in the past. I explained to the seller that I would be more than happy to sell their machine back to them but I would need to get paid for my time & parts it took to repair the machine.
So basically offer it back to them at $1050 more than you paid (plus your travel expenses). $50 an hour for your time (and that is cheap for repair rate these days) - plus cost to go pick it up - plus parts at your cost.
By doing that you at least give them a chance to buy it back if they really want it but I highly doubt anyone will want to spend that kind of money unless they fully understand the hobby.
No way I would sell it back without getting paid for my time regardless of what anyone says. In my eyes the only question is "labor rate" for your time. Typically most guys charge between $50 and $100 an hour these days so in my eyes going with the cheaper labor rate is being more than fair to the son.