I agree with novacain. Why waste time and money against something that cannot be guaranteed?. IMHO,if i were you, i would just remove the guy from my resume and leave it at that.
With such a small group of people he might not have another choice as to who to get to be a reference. And having a gap of a year and a half would most likely come up in an interview. I am currently working for a company of about half a dozen people so I can kind of see the predicament.
Is there anyone else there who would be qualified to give as a reference?