I'm sure with antibiotic he will be fine. Usually as long as they are still eating well and not blowing snot, it's caught early enough.
I don't know if quarantining will make much difference at this point or not. Chances are Sebastion was already contagious so moving Damien now probably won't help. Just mention to the vet that you have another one at home and ask if you can just phone in for meds if he starts showing symptoms too. Good luck.