Hi Coleen
That is very complicated. I will try to unpick it.
Last things first. Leave your son out of the equation. He should buy a property when he is ready to buy in the place he wants to buy. He does not want to be an owner of a property in which his parents are living and in which he does not want to live himself. If he (and a partner) wants to buy a house, he won't be able to if he already has a house and mortgage. If he gets a job locally, and wants to buy your house, then that is different. But don't anticipate it. Sell it to him if it's appropriate at the time.
There have been advantages in the past for first time buyers, although I think that they have all ended now. But it's not impossible that there may be big advantages to being a first time buyer. You don't want him to lose them for buying a house that he didn't really want to buy in the first place.
I know you say he could buy your house and then sell it again if it doesn't suit him. Moving like that is expensive. 1% stamp duty and solicitors fees. New buyers usually want to put their own stamp on a house in terms of decoration and furniture.
You want to move to a bigger town when you retire? That is fine, but don't do it 10 years from retirement. Do it when you retire. 10 years driving 60 minutes a day will be very tiring and very expensive when you are used to walking to work.
Wait until the right time to move. If your son's circumstances change and he wants to buy your house in 5 years' time, that might be the time to make the decision.
Moving to a new larger town might or might not work out. Would you consider renting there for a while to see what it is like?