Warlocks are seekers of the knowledge that lies hidden in the fabric of the multiverse. Through pacts made with mysterious beings of supernatural power, warlocks unlock magical effects both subtle and spectacular.
Emphasis Mine, from the Player's Handbook page on Warlocks, pg 105.
Going by just what the Player's Handbook we get a vague "Maybe, not sure,...
Let's start with:
I'm also presuming that this aura lasts for a turn, and isn't active for the rest of the session
The aura will be active as long as the sorcerer concentration remains up. So a newly-arrived creature that starts its turn in the aura will be affected also. Auras travel with a character and affect creatures within its area of effect.
As to ...
Unless the sorcerer is in a solo adventure (or in a 1v1 arena combat), the counter to this ability is simply to attack someone else. The sorcerer is presumably traveling with other people, so attack them instead of the sorcerer.
The Charmed condition is remarkably weak when there are other targets; Frightened is far better at battlefield control.