I'm afraid that if you gift a share of the property to your son, that share will be deemed to have been "sold" by you to your son at that time at the open market value with the consequent Capital Gains Tax implications. The only circumstance where the CGT can be delayed is if the gift is of a business asset which a let residential property is not unless it is a furnished holiday let. In that case, the CGT liability would effectively be transferred to the recipient of the business asset.
The gift would also be a potentially exempt transfer for Inheritance Tax purposes. So long as you each live for at least seven years after making the gift, your respective shares will not be included in the valuation of your estates for IHT purposes.
You should also look at the HMRC notes on property transfer and stamp duty here
to see if you are affected by the rules.
Depending on the size of the gain you would be deemed to make on the gift of a half share of the property, you might consider gifting the 50% in smaller shares in different tax years to use the annual CGT exemption which is currently £11,000 per individual per tax year. However, this process of a gradual transfer of 50% of the property could be attacked by HMRC as aggressive tax avoidance under the series of transaction rules in which case they may seek to tax you on the gain as if the 50% had been given away in one tax year.
I realise this is not the answer that you wanted to read. Let me know if you have any further questions.