Hi,
As the transfer is to your son, a gain is due on this 25%. To work out your gain, you would need the value at the time purchased and a value at the time of the transfer and take the amount given to your son. The difference is your gain. As its joint, you then split this between you and your wife. You can also deduct any costs, apportioned, that relate to the buying and selling. Any gain then due needs to be reported within 60 days of the 'sale'.
Further guidance is at:
Report and pay your Capital Gains Tax
Thank you.