High side mosfet source voltage does not switch back to ground
Capacitance of the mosfet is not the significant issue here. The heart of the issue is the poorly matached reactive components in the output filter. Any parasitic capacitance across the lower mosfet (likely mostly due to the diode or the layout) is going to present some voltage at Q1's source terminal. Not because Q1 is failing to switch off, but just because of the stored energy in the LC circuit and the lack of opportunity to dissipate it.

See the following LTSpice simulation:



Just 5nF of parasitic capacitance is enough to create the waveform you're seeing:



If I change the output values to something more suitable: L=3mH, C=60nF and R=150Ohm, but still leave the 5nF of parasitic capacitance, I get something more expected:



V(n002) is the Q1's gate voltage, V(n004) is Q1's source voltage, V(n005) is the output voltage and I(L1) is the current through the output inductor.