It’s a complete shit-show. He should have been told he can’t come without either a proof of vaccine or a cast-iron medical exemption. It will be interesting to know whether this was his team trying to bluff him in on an incorrect visa or the authorities turning a blind eye because he is who he is - probably a bit of both.
Lol at the Serbian President now getting involved trying to make it political. Kick him out and be done with it. He should never have been allowed to travel there in the first place if he did not meet the entry requirements.