You can apply for Chinese visa on his behalf in Singapore, but the requirements are same as he apply for Chinese visa in person. Generally speaking, Temporary visitors to Singapore should apply for a Chinese visa in their own country. For the special case, the visa officer has the rights to make the final decision on whether issuing a visa or not. Therefore, you can have a try, or he apply for Chinese visa in Singapore, the Invitation letter from your company and an original visa notification from an authorized government unit, such as foreign affires office, should be sent to him, the copy of Immigranting Letter Approval for Vessel and all crew details are also required for visa application. The requirements are less if he apply for Chinese visa in Korea.