Performing Poorly on Mentor's Exam
My advice is to reach out AFTER he has graded the exam (if needed).  This is for a few reasons.  


I have seen (and experienced) lower grades on an exam after a student told prof they had done poorly.  Teachers are humans and prone to halo effect bias.  If you did a bad job, let him decide that.  Don't do something to make him think that a priori. 
You don't want to exert (or be seen as exerting) any influence on the fair grading of your paper.  For example, that prof cuts you a break since you will work together.  (If you get that anyways, now, well...que serat.)  In theory, he is an inviolate Roman judge of fairness.  But in reality, teachers are people.
It may be seen, even accurately seen, as you being nervy, not strong.




If you hose the exam, then yes...talk to the prof about still having collaboration interest. Let him know your feelings.  But also be open to some real two-way discussion (even to include him saying "this would be a bad idea" or "let's see").