YOur circuit (assuming it works the way you think it's supposed to, I'm in a rush and didn't check over the 555 timer itself, just the two output capacitors and diode), it provides a flat 24V referenced to ground which may, or may not be enough (or possibly too much) gate voltage for the high-side MOSFET depending on the voltage it's source happens to be floating at during operation.
What you really want is to apply a voltage with respect to the source of the FET. Right now, yours is just a regular high voltage source. If the source of the high-side MOSFET happens to be near ground , you get 24V across gate-source which will probably fry the gate. And if the source voltage is higher (like 15V) the gate won't turn on as well as you want because now your gate-source voltage is just 24-15= 9V.