I am Malaysian, flying to Shenzhen later. My China visa just expired. Can i apply one upon arrival?

If you will only visit Shenzhen for less than 5 days, you can get one upon arrival at the entry port of Shenzhen.

My son is arriving early morning in Beijing. From Gatwick. Monday 21 october. 6 am. He has a single entry. Mistake! He needs a second one For HK. He is in Beijing for just a few days. Please advise if you can assist.

Is there an agency at the airport?

Your son needn't apply one for HK if he is a British citizen. He enjoys 180 days visa-free period in HK. If he is a Malaysian, he enjoys 90 days visa-free period.