I'm sorry you are going through this, I would talk to a lawyer and get a free consultation to see how much it would cost and what can be done.
Sometimes the age limit on child support gets extended if the child is special needs. I don't know your sons background or if there was anything put in the decree about that. Also, there are statutes to protect someone from an ex spouse from purposely lowering their income to screw you. For example if he just quits his job and takes a job that is 8 an hour, the judge can still make him pay the same amount he always was because HE CHOSE to change jobs.
People are so vindictive and take advantage. Getting a free consultation from a lawyer won't hurt you, and you might find out you have many options.