The gift to by the father of his share in the UK property to his son is both a disposal for Capital Gains Tax purposes and a gift for Inheritance Tax purposes.
As he is a long term non-UK resident, the father will have no Capital Gains Tax liability in the UK if he gives his son his share of the property by 5 April 2015. The rule for somebody who left the UK by 5 April 2013 is that if they dispose of an asset they owned before they left the UK whilst non-UK resident there would be no CGT liability in the UK so long as they did not return to the UK to live within five full tax years of leaving.
The five full tax year period started on the first 6 April after the father left the UK so he has clearly met that condition to escape UK tax on any gain. The son will acquire the second half of the property with its open market value being the cost for that share. If the gain had been taxable on the father, the deemed proceeds will have been the open market value of the 50% share.
From 6 April 2015, gains made by non-UK residents on the disposal of UK residential property will be subject to CGT in the UK but only on the difference between its value on 5 April 2015 and the disposal proceeds.
So long as the father lives for at least seven years after the date of the gift, the value of that gift will fall out of his estate for UK Inheritance Tax purposes. Otherwise, it will be included in his estate but may be subject to taper relief which you can read about here. You might also be interested in the HMRC page on who pays Inheritance Tax in different situations which you can find here.
I hope this helps but let me know if you have any further questions.