Since you are supplying an unbonded panel through a switch that isolates the neutral you should bond the neutral to the generator frame and provide the appropriate grounding electrode system for the derived system that is the generator. You can run an appropriately sized Grounding Electrode Conductor (EGC) to the existing grounding electrodes for the utility service equipment.
If the service equipment is by any chance SquareD then they make a kit for interlocking the main breaker with the position 2-4 breaker. That would not switch the neutral but if you can unbond the generator neutral from the frame and use the service equipment main bonding jumper as the bonding point you will save a lot of money over the cost of three pole transfer switch.
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Tom H
