If he already has a room assigned, you can simply forbid the door if he wanders in there. Anywhere with a door can be a potential quarantine room. It might not be ideal, but it'll give you time to get something set up depending on what you want to do with your new were-citizen. Are you looking to purge him? Weaponize him? Throw him into an arena for entertainment?
For future such attacks though, it's always good practice to assume any dwarves fighting a werecreature were infected. Getting them quarantined for the next full moon is priority. If you have time and patience, you can dig through the combat logs to see who got bit and who didn't. I don't have the patience for that most times, so I just assume worst case scenario and get anyone involved locked up in a room. Separate rooms. You don't want the infected attacking your clean citizens. Besides them also getting infected (or killed), there's a chance of a loyalty cascade happening if the were transforms back to dwarf during the fight.
I get my dwarves assigned to their own rooms fairly quickly, so I usually just lock them in their rooms if the medic has already patched them up. My hospital is also set up with separate rooms for the injured, with a door on each room for just such an occasion, as well. A central room for just hospital supplies and a well, and depending on my mood, several 2x3 or 3x3 rooms on both sides, each with a traction bench, a table, and a bed between them.
It's as simple as forbidding the door to keep potential were-citizens from wrecking havoc. Anyone that doesn't transform is let out. Anyone that does... well... the dwarves of Goldenspark don't suffer monsters in their midst. :3
Hope this helps!