A very difficult situation!
The thing is that it is no good saying that the halter was taken off and he was beaten with it. if he was let loose then he would have run away so apart from one whack the person would not be able to beat him.
I would say that he does seem to be frightened but this fear is of his own making.
What to do about it is difficult to say.
You say he has been tranquillised but obviously not enough!
I would go to a good horse vet and ask for some ACP tablets. Give him several so he really is doped up preferably if you can get him in a round pen of tall metal panels that he cannot knock down.
Once he is really doped you should not have any problem getting a halter on him but use a good long strong rope and I would also use a control halter.
You get the RSPCA involved using a dart gun seems to be extreme.