Thanks for the contributions folks.  A few thoughts…

Terry said:
"You can only claim rent at market rates, transfers are at market rates as are CGT and stamp duty calculations."

Claim what?  I'm a real newbie.  Could you spell it our for me please?

If I transfer it to S & A in a few years, for the balance outstanding on the mortgage, will we be hit for duty on the market value rather than what they "owe" me?  That rather diminishes the value of this arrangement.

"You could also buy it now in your name with you are his/her trustee."
I feel a visit to a lawyer coming on.

"Also family law issues – what if you or he get divorced and spouses make claims etc."
Don't think I dare think about this.

Glen said:

"Is there a possibility of going guarantor on the loan or doing a family pledge to help your son?"
What's a family pledge?  Have to say I prefer the security of owning the property myself.  If it does all go belly up at some stage, I have the firmest control over my assets.   Seems the worst  for them is that they'd have been paying rent to me for the duration of the arrangement rather than to a stranger.

Buying another property in a year or so is a real possibility – I have another son…..

Illuminati said:

"the setup you have describes it sounds like a lease to own.  Check out "wraps" where they can "rent" at a rate that covers your mortgage and can pay out the property value at any time"

Very interesting.  I've never heard of this.  Any clues on where I can learn more?

As I said above, I feel a visit to a lawyer coming on.

Many thanks for your comments.

Robyn