Whoa. Cease and Desist! Your action plan is to have no action plan. Your candidate still works for a client that paid you money. A CV on Career Junction is not quitting any more than sleeping on the couch after an argument with your wife constitutes a divorce [That comes later].
You have an unhappy candidate who has decided after 8 months he wants to leave, and he has begun that process. You should not aid or abet that process.
[You don't tell me if the candidate called you and asked you to place them somewhere else. Some recruiters will tell you that in that instance it is ethical to work with them, but I have never been able to sell that to myself, let alone a client who paid me R120K and accuses me of stealing. If I placed you, until you leave, you are "untouchable"].
Now, for the love of the Recruiting Gods, DO NOT contact the client to see if they will do something to keep him. Should they react negatively and fire your candidate, you have just exposed yourself to a world of legal liability and pain. That is an unfair dismissal suit waiting to happen!
There are not many times, as a lifelong Type A, that I give this advice to a Recruiter. But in this case, put your feet up on your desk, and do absolutely nothing!!