Renewing/re-issuing M type visa

My son has an M type, with 2 entries. The current one was issued in the US. It expires Dec 20, with the last entry date of Sept 20. He is in China now, working. He needs to return to the US for 2 weeks in October, and then go back to China to his job. However, his 2nd entry on the current one will be used when he goes to Hong Kong next week. Can he get a new one in China that will allow him entry for his to his job in October? Or must he get a new one when he is back in the USA? That will be difficult, as the consulate for our state is located in San Francisco, and his home is in Alaska.