Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
If his name comes off the flat (though sale or gift or a bit of both) then you will pay the additional 3% stamp duty as one would assume you already own property (and clearly will not live here yourself) along with any usual rate of stamp duty arising
The same applies if he remains holding an interest in the flat -
Then you/all of you will have a capital gain position to consider when the property is sold, as it will then have been none of your main residences (and for your son more than 18 months after moving out)
If your son gifts the remaining share of the property then he also has Inheritance tax to consider, should he not survive more than 7 years from the date of making the gift.
Let me know if I can assist further
Yes he will I am afraid (so stamp duty accrues on all of you even if he only gifts 50% as he will have a new pucahse too - leaving this property to become the second home)
Even if he did not buy a new place straight away the stamp duty additional charge is applied to the property not the names purchasers/owners
let me know if I can assist further, or if you have all that you need, it would be appreciated if you could rate me for the level of service I have provided (or click accept)