He cannot voluntarily reduce his income and then seek to reduce child support. If he gets fired due to his own misconduct, he cannot seek to reduce his child support. He should be held to salary he was making before he was fired. Make sure the court knows he got fired and that the child lost health insurance.
Likely not. We do not know the burden of proof because you did not tell us the details of the current order of support. Nevertheless, the mere act of being fired is not enough to change the order. I do hope you are not representing yourself. He may be commencing a pattern of litigation coupled with his employment change. Kids are the new oil.
If he voluntarily decreased his income, he can be held to the same income he was making before he was fired. Consider going to a lawyer to get a more in-depth perspective on your case. Good luck!