I'm paranoid about biosecurity, and never introduce outside birds here, only chicks from very good hatcheries.
If you visit this bird, see how he's acting with the other birds, and with the flock owner, and especially with you as you walk around him. If he's caged and 'can't be out' that would be a red flag to me about his behavior. Human aggressive jerks don't need a new home, they need freezer camp!
Then actual quarantine is difficult at home! Do your very best, because it really matters. Can he stay at a friend's place, where there are no chickens, for three weeks, so he's not exposing your flock to any issues? I'd treat him with permethrin once or twice for miles and lice anyway, at least.
Then, at your coop but separated by wire for a few days or a week, and introduced during the day outside. Single roosters are the easiest to introduce usually; much easier than single hens!
Good luck,
Mary