I'd recommend waiting until it begins to get dark and going out to watch where he roosts. Birds are much easier to catch during the night. After he is captured, you can decide what to do with him; you can eat him, try to rehome him, or keep him.
If you decide to keep him you should definitely do a quarantine period, even if he has already interacted with your flock; not all diseases will be spread quickly or from just a few moments of contact. Keep in mind that the chances of him having any disease are low. Not by any means impossible, but not necessarily common either. Quarantine is of course a very important safety measure and should always be used as a precaution, but it is just that, a precaution, and not often does it prove to be more than that. Keep a close eye on your flock the coming few weeks anyways, but try not to worry too much.
If you don't decide to keep him, you should keep him separate in the meantime while you determine what to do with him.
