You are liable for CGT on any increase in the value between the date you moved out (into your new house) and the market value today -less any unused annual CGT allowance you are entitlede to in the year of sale to your son. Note though, that provided you sell the old house within 3 years of moving out, no CGT liability arises.
The rest of TW's answer (regarding IHT liability if you transferred to your son at less than the market value and were to die within 7 years of that transfer) still stands.