The larger and immediate context begins at Luke 9:46 where the apostles are arguing about who among them is the greatest. Verse 49 is withing that discussion. Whilst being corrected for trying to see who was the greatest among themselves, John asks about someone outside their immediate group.
Jesus often talked with large crowds, many of whom may have listened and put faith in him, but were unable to leave house and home behind and follow Jesus everywhere. The man doing these things may have been among them.
It seems to me that this is just a continuation of verses 46-48. It is more correction for the attitude of the apostles. To focus on the man from outside the apostle group is probably to miss the main point of the passage. It was the apostles whose attitude needed correcting.