Gifting flat to my son

Yes. You have to register the gift deed and also liable to pay the stamp duty on gift deed (if applicable in your state)  

U registered a Gift Deed in favour of your son for your 1/2 share of property. OR U registration a sale deed of your 1/2 share property in favour of your son. If property in Gujarat the stamp duty and registrarion fee is same  for both document.