He likely needs some pain meds in addition to the antibiotic. See what the vet suggests.

I am sorry this happened to the 2 dogs. When 2 dogs can not be together, and they are in the same home, it really is awkward.

Water doesn't set well on an empty stomach. Is he eating? Seems canned / moist food work better just now to get fluids in him, can you do that?

The cold may help numb pain. Or the water may be going in the cavity and causing sinus pain. Ask the vet about that too.