Although child support may be "due" at the beginning of the month or on some other date, it is not considered delinquent until the end of the month. If the full (increased) amount is not paid by the last date of the month, you can make a demand for the shortfall and determine whether enforcement action is necessary.
** This response is provided as general information only and does not constitute legal advice or otherwise establish an attorney-client relationship. **
You should review the Order that modified child support. Generally, the Order sets forth a date that the new amount of child support begins. If your statement "he was told to pay the rest of the amount this month" then the month is not over so he may not be late. Usually the Order would set forth more definite terms.
Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.