what to do it Father doenst pay on time?

Is he paying through the court system?  Or you directly?  If he is paying thorough the court system then the SCDSS child support division can have him ruled in for contempt.  If he is paying you directly then you can file a contempt action.  If he is found in contempt then he will have to pay your attorney fees.  If you file a contempt complaint you also need to make sure your future child support payments are paid through the court system ( if he is not already) so that SCDSS can monitor his payments and you will not have to worry about it.   It does not matter if he wants to pay less.  He has to pay what is court ordered.  If he wants to modify his support he will have to file his own action.  It will be lowered only if he makes substantially less then he did when the court order was made.