Why do the high side MOSFETs get hot in this buck converter?
the IR2110 high side driver simply isn't capable of providing enough drive current to drive all of these MOSFETS in parallel. Each mosfet has an input (gate) capacitance of typically 7000pF at turn on with the 10R gate resistance (ignoring the diode) the required input current is initially > 6A - the IR2110 is only rated for 2A - you will be getting a very slow turn on of the MOSFET's, hence lots of heat. Try one IR2110 per MOSFET