I'm afraid that you son can't get a dependent visa to go to Beijing with you. But he can apply for a Tourist Visa for family visit, when you fill the visa application form for him, please asks for 180 days stay duration for your son, the final decision is up to the visa officer, if your son just obtain a single entry L visa with 30 days stay duration, you can apply for visa extension at the Beijing Exit & Enter Department one day before the expiration of the visa, L visa can be extended twice, then your son can stay in China for 90 days in total.