Some of these responses make me laugh! OP has invested very wisely on behalf of her son (as directed by the grandfather) and made her son a good deal of interest whilst acting as a his 'fund manager'.
How is investing in property directly any different from putting the money in a bank or savings account? Do some of you think that when you put money in a bank it just sits in a vault waiting for you to withdraw it... ??
That money gets invested by the bank into ventures such as... an individual loaning money from that bank to... do a loft conversion for example. That's how the capital gains interest.
Suggesting that the OP has defrauded her son in some way is ridiculous. Investments take many forms- traditional bank accounts, stocks and shares, art, property, etc. OP had a great investment opportunity and took it. Property investment of this type is typically low risk and hight reward - as evidenced by the outcome and interest it has made.
OP, if you want to be completely accurate and equitable, you might look at how some property funds have performed over the time period in question and calculate a figure from that. This would be a proportion of the interest made - similar to how property fund managers would pay a proportion, factoring in their costs and payment.
Before doing anything, I'd talk to your son and ascertain his wishes over where he plans to invest this money going forward. Leaving it where it is may be the best financial option and the option he wishes to choose when the money becomes his. Seems like you have got it invested in a cracking 'fund' at the moment which may continue to perform well for your son. Seems pointless to withdraw it to put it in an inferior investment opportunity. In ten years time, he may choose to release his equity from your property to buy his own home - it may well be worth much more than his original 10k stake and set him up beautifully.
Perhaps take this as an opportunity to teach him the various investment options available- the risks and benefits of each, map out some best and worst case financial projections of each option and let him decide. They important thing is that he gets to decide (with your guidance) and have autonomy over where his capital is invested. You've made excellent decisions with it so far - hopefully he will follow your excellent financial decisions and make good choices.
Good luck!