These arrangements make no sense whatsoever.
If the price doesn't rise to price you agree, he doesn't take up the option to buy.
If the price rises, he exercises his option.
You are stuck in a property which you can't sell for 15 years.
Even if he has a contractual obligation to buy it, it would be very difficult and costly to enforce.
If he wants to buy the property and you want to sell it, sell it to him now at the market price.
If he can't buy it now, tell him that you will let him know if you are selling the property and you will deal with him then. I think it's good manners and good business to keep a tenant informed. So if you are thinking of selling, let him know. Try to give him as much notice as possible.
Brendan