Someone has hacked my 13 year old son's Facebook account and has blocked me on Facebook and messenger. How do I get this unblocked.
The owner of a hacked/compromised Facebook account needs to go to https://facebook.com/hacked and follow the instructions to take back control of their account. Only the legitimate owner of the account can do this.
If you get blocked by another user, you can't ask Facebook to remove the block.
However, have you considered the possibility that your teenage son blocked you willingly because he feels uncomfortable with you seeing the stuff he shares with his teenage friends/girlfriend/crushes, and then he made up the "hacked account" excuse? I think it's the more likely explanation.