He need to apply for an F visa to China. what you said the invitation letter is the requirements No4 as following needed documents for An F visa.
1.One visa application form. No item should be blank .
2.One recently taken 2X2 in. photo showing entire face and without a hat on. And affix the photo to the application form.
3. Original passport with at least 2 blank visa pages and valid for at least 6 months beyond the date of application.
4. Visa conformation letter from an economic and trade bureau or a foreign affairs office of the provincial or municipal government, which proves his purpose and time of stay in China, etc.
5. Non-Singapore citizens shall provide documents that entitle them to reside for an extended period of time in the Singapore, such as a PR, WP, EP, STP, DP.