Unfortunately, if either parent is earning more or less income then there is a material change of circumstance that would justify a Motion to Amend. However, if your ex is making less money because he was fired then his former income should be imputed to him. Your ex cannot be voluntarily underemployed.
If your ex attempts to calculate the new child support based on his lower income then I strongly suggest seeking counsel. Certain specific information is needed to satisfy the imputation requirement and have the judge impute income.