In the scheme of things, you helped with allowing him to have that vehicle for a year. You allowed it. No other way to say it. I doubt you have documented your conversations. Plus, you have to prove mileage when you gave the rig to him and that he put the miles on the rig and get a readout from DIC.
Last but not least, do you really want to tie up your time and money taking a 70 year old man to court? For all you know, he may have medical issues that prevent him from timely work and having cognizant thinking in his daily actions. I would just chalk it up to a learning experience and how to proceed from here is your call. You are going to have to show that the old guy was doing something illegal and have expert analysis on your behalf to back that up. Otherwise it is just your word against his and the fact you left the rig with him for a year is going to go against you. Plus, if you do all that and win, will he be able to pay you what the court may award you?
Maybe Judge Judy will pick this up and pay the both of you to appear on tv.