Child support modification question in missouri?

My child is only 10 months old and just started receiving court ordered child support from his father (Like last week was the first payment after waiting on a response from him since November). They finally garnished his check in the amount of 395 every two weeks and now he's calling (he has only seen him once) trying to get me to get it lowered saying they got his income wrong. Now, I'm not the courts and I have no idea how they got his income, i'm assuming it's from his job themselves but can he get it modified after one payment? I'm not on TANF or any other state assistance and I've been 700-plus in daycare plus food, clothes, shoes rent, gas, electric, etc on my own so it's nice to FINALLY get something to pay for daycare. Can he get it modified after only one payment or does he have to wait the 3 years before he can do so?