My son has been offered 2 weeks unpaid work experience in China. What type of visa is required?

My son is a 16 year old student who as part of his school course needs to complete 2 weeks of unpaid work experience. He has an opportunity to work with me for 2 weeks in China. I am a consultant working on an F Business Visa for a Chinese company. My client is happy for my son to spend 2 weeks working unpaid in their office so he can practise and improve his Mandarin. What type of Visa should my son apply for? Is a tourist visa sufficient? Is an invitation letter also required?