Boost converter design help
I am able to get a 250v output but if I add a load to it the output
  voltage drops drastically.


A standard boost regulator circuit transfers energy each switching cycle. It isn't equivalent to a transformer and so loading effects change the output voltage considerably. For instance, during the period when the transistor is on, the inductor is effectively grounded and current rises linearly to some value. You have "charged" the inductor with energy: -

Energy = \$\dfrac{L.I^2}{2}\$

When the transistor opens, this energy has nowhere to go but through D1 to the output capacitor and load. If you store 0.1 mJ and then release it to the load/cap and you do this 100,000 times per second, you are actually forcing a power of 10 watts to the output. Clearly if there is no load current, the output voltage keeps rising (due to charging the cap) and after a short period of time you get smoke.

To turn this system into a voltage regulator requires that the duty cycle be controlled by an error amplifier. In other words, to keep the output regulated on virtually an open circuit requires that the duty cycle is almost zero. As load current increases, the duty cycle MUST increase to maintain good voltage regulation.