You might refer to HS283 which deals with the man residence and CGT.
Your son may have qualified for absence relief as it is impractical for him to live in Watford and work as a paramedic in South London on shifts. However, he would need to re-occupy the property before selling it to qualify for that relief. He could try to claim that his job still prevents him from living in Watford but as he can claim letting relief instead, there is no need. His CGT position is as follows:
Gain £41,000 (£220,000 - £138,000 / 2)
Total period of ownership: 133 months (to November 2016 say)
Total period of owner occupation: 81 months
Total period of letting: 52 months
Exempt gain: £30,519 (£41,000 / 133 x 99 (81 months + last 18 months)
Non-exempt gain: £10,481 (£41,000 / 133 x 34 (52 months - last 18 months)
Letting relief: £10,481 (lesser of £40,000, £30,519 and £10,481)
Net taxable gain: £0
Your son should have no CGT to pay.
The only relief you will qualify for is the annual CGT exemption of £11,100 so you will have a net taxable gain of £29,900 (£41,000 - £11,100). There are two rates of CGT on residential property, 18% and 28%. The rate or combination of those two rates that you will pay will be dependent on the level of your income in the tax year the flat is sold. So, the CGT cannot exceed £8,372 (28%). That would be the case if your gross income was £43,000 or more.
I hope this helps but let me know if you have any further questions.