can you pls advice on what to do? he is stranded in Beijing now.

Giji, it's true that British people do enjoy 72 hours free transit in Beijing. However, his route isn't qualified because his flight stops in Shanghai too. The 72 hours transit policy requires that passengers can only fly in and out from one city where this policy has been implemented.

In his situation, he will be required to take a flight from Beijing to Britain. According to what I know, the airline that allowed him to board is responsible for this. The immigration officers might ask the airline to take your brother back.