You'd have to call him. He isn't going to make the case any worse and there is a really good chance that he will get the jury's sympathy.
His resignation was not an admission of 'vigorous treatment' The judge would not allow cross-examination on it, since nothing was proved against him. Even if that were proved, it is unlikely to be adduced since it doesn't prove anything in the instant case. That a man is guilty of an assault in the past doesn't tend to prove anything in any later trial for an assault. It would be different if that man brazenly said in evidence that he'd never raised a hand against anybody, ever, in his whole life. Then it would be adduced to show he was lying about his good character.
Incidentally, he doesn't " take the stand" This Americanism isn't applicable to Britain We don't have a 'stand' for the witness to take.