There is not much you can do unless you have anything in writing confirming the agreement with the stepson.
Do you have any written agreement or written correspondence?
What about the solicitor who acted on the purchase? Would the solicitor have any records of the arrangement?
The firm will have the file. They only get destroyed after 6 years. Ask the firm for it. They cannot refuse, it is your file.
You may not be able to force a sale but you may be able to put something on the title that if it is sold you get the money. It will depend on what you can establish from the file. If it ends up looking like a gift then there is little you can do. First step is to get that file and see what is stated within it.
If they solicitor failed to advise you properly that is negligence and you may have a claim against the firm.