I am not sure if you can get a UK number in Hong Kong, as in having long term monthly payment plan ones.
I used 3 in England, so I am no sure about other telecom companies' practice, but your boy would only need to bring his passport. He can just walk into a 3 store and tell the staff what kind of plans he prefers, the one I had... as far as I remember was like 12 pounds a month unlimited data.
As for whatsapp, you only need internet access, there is the option of not changing the number, so your son can keep his HK number for whatsapp.
He can just get a pay as you go sim card in the airport when he arrives England and then top it up, and then go to a telecom company to setup a long term one,
我英國留學,都係兩機,一部 keep 香港 number 同 wifi WhatsApp,另一部 UK 電話 (當地求其一間公司),除聯絡銀行,診所,政府部門之外,無其他用處。
UK 買 Sim Card 好簡單,就咁去電話公司連鎖店買就得。UK 有 Pay as you go,要 top up 就去超市買。如果要上台有 contract 的話,需要 proof of ID, uk address, school acceptance letter 同 bank letter。我冇上台所以唔知邊間好。