15 years is much too long. You owe him no duty. My guess is he is looking for a character witness, and those who have known him the last 15 years must not be any help, since you are the first he called.
If he is innocent, then the best advice any lay person can have for him is the advice you gave him. Keep his mouth shut. Hire a lawyer.
"This brings to mind the woman who accused a catholic priest in California last week. The LA Times reported that the priest molested her 32 years ago. The priest catagorically denied everything and said bring on the investigation.
Now, 1 week later, the reports are coming out that the Times did not do any investigation. They took her word for it; she contacted them."
Accusations like these could be brought against honorable, more or less powerful, people for political ends. This is an unfortunate thing indeed. Wise people, who find enemies at their door whom they believe to be capable of such deception, might heed the sage advice of Shimmer's husband, extolled in Shimmer's post 162: "He won't let me leave the house if one of our daughter's friends is over. Because he also believes that if you are accused of something like that, regardless of whether you did it or not, your reputation is forever tarnished."
Nonetheless, I think he is guilty as hell and I wouldn't anser the phone if he called. Sorry.