Question about duration

My son has a one year visa to China (good until April 2014). It is a category "L" , with mutiple entries("M") , and with a 90 day duration of each stay. He is in China now and got his L stamped when he went to Hong Kong about two months ago. This means he will have to leave China again by the end of the month to meet the 90 day duration requirement. My question is, can he go back to Hong Kong to get it stamped again or is there some kind of limit on using Hong Kong more than once to meet the requirement of leaving China?