What is the full definition of Custodial interference?

I went on vacation for 1 week and 1/2 and asked that my sons father watch him which he agreed. When i came back from vacation he refused to bring him back to me coming up with crazy allegations and fabrications about my parenting. I called the police to ask them to help me because i clearly have full custody but i was told that since (there was not specific dates of visitation in my custody agreement) that they couldn't interfere and take my son away from his father. My sons father then said that he WAS taking him for four weeks for a summer vacation (which was never discussed prior) and that if i called my son then it would be considered "custodial interference" and that his "two weeks" of uniteruped parent time would start over the following day because of me calling. Is this true or is this a complete bluff because it just doesn't seem right in any aspect? How do i go about getting my son back and how would the "keeping of my son" look on his father's behalf in court?