My son's mother has not returned him from his weekend visit. Can I have my parenting plan enforced?

Absolutely.  If you have an address or other contact information for the mother, you need to let her know, through every channel you have (email, voicemail, snailmail, etc.), that if the boy isn't returned, right away, you are going to come after her, legally.

If your son isn't home by Monday morning, I'd hire a lawyer and file papers to have the mother declared in contempt of court, for violating the parenting plan:  that parenting plan is also a binding, legal court order, and she can't get away with this.