You will have to request that the Final Judgment of Dissolution be modified to award support beyond your son's 18th birthday. However, once he hits 18 he becomes an adult under law, which does not favor additional support.
Is he sending th correct amont? It is going to be difficult for you to get chld support beyond his high school graduation, even if you can tie the year delay to the disease. Even though it is an uphill battle you may als wish to consider adding a count to modify the judgment to get yourself an award of some alimony on the grounds that your financs have radically changed so long as you are his primary caretaker, and respnsible for his medical bills. Before you get too deep into this, however, you should determine how much money your ex husband has/makes. If he doesnt have much you could win a drawn out and costly legal process and lose by getting a very small award. Is there any chance he would help out voluntarily?
Yes and Yes. (If you move now while the child is dependent.)
Many final judgements have the high school language in place. Does yours?
Asperger's could extend that further.
You need to speak with someone locally now, because once an obligation is gone, it is gone forever.
Finally, if he owes support, he owes a vested obligation to you or your son.
Child support can be extended for an adult child who is dependent in fact because of mental or physical health issues. The extension is not automatic, though, so you should consult an attorney on how to proceed, and do it before the child turns 18. Normally, though, child support continues until the child graduates from high school, unless the final judgment or settlement agreement says otherwise.