First of all, you should start your answer positively. Mention strengths, good things. You can say he is very skilled in interviewing people (most recruiters believe to be like that, doesn't matter if true or not). Or you can compliment him for something else you observed, for example being detail oriented, well prepared, and so on. It is a best way to start answering this question.

Once it comes to the weaknesses (second part of your answer), try to take it lightly. You should mention at least one weakness, because that is your task. However, I would suggest you to pick something light, like being little impatient, over-friendly to job applicants, little confused, or something like that.