Am I Being Unreasonable? Original Owner Wants Car Back

The fact that the OP was not looking to sell his car period, but realizing what it meant to the original owner's son, decided that he would consider selling, under his terms seems to have been lost in the 100+ responses. The OP has invested time, money, and labor into making this what he wants. It is neither unreasonable or unfair that he expect to be compensated for what he has put into the car that he didn't want to sell to begin with.
At this point, if it were me, I'd would simply ignore any further inquiries or requests from the son and enjoy my Demon. If he decides to sell it at some point down the road, it's value will be determined by what the market will bear, and the original owner's son can continue to wait and watch for it to become available on the open market, potentially realizing some savings on the price at that time as well.