Writing Recommendations

How do you write a professional recommendation for a former employee who failed to make the grade? A nice young man I used to work with was my direct report. I had no issues with him, but my boss did. His attitude was horrible, he was even moodier than I am (and he doesn't have PMS!) and often dragged the rest of the division into his difficulties. He's the only one I ever had to take to HR. I left the company almost a year ago, he left this summer and wants a rec from me. How do I give him an honest review without hurting his chances?

It's supposed to be my day off, so why am I doing this?