China will do nothing for the Philippines.
Geopolitics is about geography influencing local politics. Once the Soviet Union collapsed, the Philippines ceased to be geopolitically significant. The island nation is too far from mainland Asia. For the US, Viet Nam is far more valuable.
China is too powerful for the Philippines for the Philippines to even try to arrest China's claim to the entirety of the SCS, so what else can Duterte do but to publicly submit to China ? But the problem for Duterte here is that China do not need the Philippines to control the SCS. China can slip the Philippines some aid here and there, but nothing comparable to the US. If we leave the Philippines, so will China, and everyone doubt that Duterte have the intellect to run the Philippines effectively.