Validation Problem

I am building a form with a checkbox and a dropdown that change its values depending on the value of a checkbox. For example when the checkbox is selected an invoke command calls a hidden command that calls a web service to fill out the dropdown. Since the dropdown is required I selected a property called required, the problem then was after a post back was triggered by selecting the checkbox a required warning appear beneath the dropdown. That is why I set to true in the hidden command the option immediate. The problem now is that the dropdown did not change its values after the checkbox was selected.

How I can make the dropdown change its values without triggering the required validation?