[spidey]

The only safe thing to do is keep them separate - crate him while the others are out, crate them while he's out. Same-sex dogs will often fight, sometimes to the death, so this actually may not be something that will ever change, unfortunately, you may always have to keep them separated. Are he and your Boxer neutered? This won't solve the problem, but it may take some of the edge off (and there's really no reason not to neuter him, unless you plan to show him).