I want to apply for a 90 day visa to visit my son

I've visited my son who teaches in Shanghai on two occasions, both less than 30 day on an S. I have now retired and want to spend longer there with him but have been told I cannot get an S again - my son's present work contract runs to the end of September 2018 but his visa only till May 19 due to his school awaiting renewal of their local business licence. In the meantime he will be issued a temporary permit but I have been told I cannot apply for my visa on this. Should I apply for a 90 day tourist L even though I don't have any travel bookings? I leave on May 13.