Tough situation.
A basket muzzle is a great choice for now while you consider other things.
I’m not anti e-collar, and would go as far as to say that in this situation a hard correction might be useful (even without training, which of course is ideal, but…in this situation where it’s not installed yet, I might still try). But I definitely like the idea of the muzzle BEFORE trying that.
That said - I have 3 unneutered males, and they take some managing. And I don’t consider it out of line for any one of my dogs (or all 3) to be absolutely unwilling to accept a new unneutered male being thrown into the mix. BUT. None of my dogs have the capacity to kill another dog (or person). So I would take that into very, very serious consideration here. If you are not sure the dog is in control, and/or that the behavior is not more than just new dog bristling at each other…I would consider euthanasia.
And I’d feel terrible about it. But I’d feel worse if he killed my dog, or if I rehomed him and he hurt/killed something else.