WOW, I believe that your Husband needs to take another look at this situation and must actually stop it immediately. He needs to confront this RAV and let him know that Hashem does NOT discriminate and no one knows who he will test next with which nisayon. My mother a”h used to say “ver hut kinder in shteib zol shvagen”, those who still have children in their own home should stay silent.
This was a wise way of saying NOT to judge and “al tiftach peh l’satan”, don’t test the satan and invite him in. When a person chooses to discriminate against a child for these very foolish reasons and hurt a child like this, he has no way of knowing how Hashem will punish him. This Rav’s job is to give your second child MORE chizuk than he another child would normally get. Your son needs a true friend in a Rav, a true role model and leader to follow so he does NOT get hurt and he does not go OTD like his brother. HE should have pulled him aside and ask him how his brother is doing, and if he has some understanding what exactly turned his brother off, so he should be extra careful that the same doesn’t happen to him.
That is the position that your husband should take, that is what your husband should say when he goes back to speak to the Rosh Yeshiva and the Rebbe.