At thirteen, he's old enough to know more. I suggest you help him learn to THINK and EVALUATE on his own, as a general protection against ALL cults, rather than trying to forbid. Here's my notes on how to introduce reasoning at his age level:
Provide love and acceptance of your child’s choices, no matter what. Protect them from the truly harmful (drugs, unprotected sex, reckless driving). Have them reason with you why these things are inadvisable (or if they try and prove they aren’t show where their reasoning is flawed). Honor their independence, and express confidence in their choices. Hide your fear, they are terrified of growing up already!
I suggest you get the critical thinking booklet series from www.criticalthinking.org Work through some of the exercises together. Then your son will be able to see through your family's machinations, even if you can't supevise him 24/7.
I agree, you are well within your rights to tell the relatives to BACK OFF until he reaches the age of majority. THEN he can decide for himself.
By the way, I raised my two children alone, far away from their violent and abusive father. Even so, we talked about visitation one day IF THEY WANTED TO. I promised if I did arrange something, it would be supervised, and I would make sure they were safe. Neither of my children expressed interest until they were adults. When my son was about twenty, he arranged to meet his father without involving me. After his first visit, he said, "It was weird. He was sort of remote or something. But he was exactly as you described." My son's curiosity was satisfied. My daughter to this day has not expressed any interest in meeting her father. But I earned my children's enduring respect for honoring their CHOICE.