how to stop child support garnishment?

My son turned 18 in early December. He has all the credits he needs to graduate high school, and had a part-time job until the end of the year. Now he is taking some electives in high school, so he can graduate with his class in may. He intends to go to college, he says, but he hasn't applied anywhere. He intends to get another job, he says, but he hasn't found one yet.

I looked it up online and I know I have to pay child support until July 1st after he graduates high school, or until he turns 19 or drops out of highschool. He will graduate in may, so I guess have to pay until July 1st, even though he's just taking high school classes to kill time at this point.

The thing is, I don't know how to actually stop the child support payments from coming out of my paycheck. I'm afraid I'll have to go back to court and petition for a reduction, and they will have to go through the whole song and dance about how much money I make, what can OI afford to pay, and so forth, and figure out a new child support ammount for my two younger children, and that will take 6 months like it did last time. I still owe back child support, so I might not get any reduction whatsoever. They might just apply the extra I'm paying for my son toward my arrearages.

I wouldn't mind paying off my child support arrearages, but beacuse of my second wife left me just before we filed out taxes for 2011, and didn't allow me to claim her son as a dependent, I ended up owing a surprise $4000 in taxes I didn't expect. I paid half of that back, but, I also owe $4000 in taxes for 2012. My child support payments are $1800 per month now, and I only take home about 30% of my paycheck, and with me still owing the IRS $6000 in taxes, I'm really in trouble financially. If I pay everything I owe the IRS by April 2014, and continue paying the child support payments I am now, I'll only take home about $900 a month out of my paycheck until April 2014, and $1900 per month thereafter (30%). I don't have any savings left, and I had to file bankruptsy after my wife left me last year, so I have no credit. All I have is a 12-year old car and a house that my 65-year old mother and I live in (and that I pay $600 per month on for my half of the mortgage).