Irrespective of whether they hand the house over to him or not, they are liable for CGT at 18% on the difference between what the house was worth at the point when they started renting it to the point when they cease to own it.
Note that the point they started renting it is important, NOT the date and price they paid for it - so not the �31k figure necessarily.
This is assuming they were living it beforehand - one's private residence is not normally liable for CGT.
They would be able to offset some Annual Exempt Amounts against the 18% rate.
At the point they gift it to him, another issue is created - a Potentially Exempt Transfer (PET). This means that a period of 7 years then has to occur before the house would be free from inclusion in the estate in the event of death of a parent from Inheritance Transfer Tax - always assuming IHT threshold was exceeded (it may not be - depends on other assets).