That will give the N-channel mosfet a better turn on, but doesn't solve the problem of turning the mosfet off.
When the output of the 555 turns off and the gate voltage goes to ground, the inductor voltage reverses. This takes the source pin of the mosfet negative, so the mosfet turns on again. This effectively camps the source at the Vgs level below ground until the mosfet turns on again.
The easiest way to do this type of converter is with a p-mos device. To use a n-mos device, you need to either use an isolated gate driver, or connect the 555 so that it's ground is common to the n-mos source, but not common to the input ground.
polashd, what is your reason for wanting to use an n-mos instead of a p-mos device?