Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
As long as there are no mortgages on the gifted properties, and you do not make use of them or continue to benefit from any rental income, then there is no income tax on the gifting of the properties but there will be capital gains tax to consider
Capital gains will look at any increase in value from the time you purchased to the time you gift the properties.
Also the consideration of Inheritance tax needs to be addressed which is where the 7 years comes in. As long as 7 years have lapsed between the date of gifting and the date you pass away - then this are excluded for Inheritance tax purposes.
And of course any gift of transfer would see your son needing to declare rental income to HMRC.
let me know if you require any furtehr assistance or wish me to expand further