Do not soak in bleach solution.

Is it pretty clean (skin, meat, etc...), it sounds like it is. If so, pick up some bandage gauze and high volume peroxide (the stuff hairdressers use, not the other stuff). Wrap the skull with the gauze and then carefully wet it down. Be sure the gauze is not touching the antlers. Do not get it on the antlers, your skin, eyes, etc...very dangerous. You can buy a whitening kit from Cabelas, but this is essentially what comes in it.

Use trial and error to get the appropriate shade of white, but it will get very white.