Welcome to Fluther.
Your question is somewhat ambiguously worded. “Rejection of your son” can mean your rejection of him, though I gather from the (limited) context that this isn’t your intent. Also “after leaving his dad” could refer to his leaving your dad, but I don’t suppose a 16-year-old would leave his dad… and then have his dad not allow him to see you.
So… if I answer what I think you’re saying here: You left your son’s father and he retained custody of your son, who you think is now rejecting you, but the boy’s father won’t allow the boy to see you.
If this is an accurate description, then how do you know your son is rejecting you? That’s putting the worst possible light on his potential attitude, since you can’t know this if the boy’s father won’t allow him to see you.
Do you have an attorney? You need one, if you’re trying to see your son and the boy’s father is preventing that. And until you know that your son is rejecting you without any assistance, encouragement or coercion from his father, I wouldn’t impute that motive to him. You’re not being allowed to see each other; it doesn’t sound like the boy has anything to say about it.