Some good advice listed here.
I like the idea of a certified letter, followed by an ad in his local paper giving him 30 days and then sell it.
Take his funds and put them into a savings account. If he ever contacts you back you can say in good faith that you tried and you can give him back his money (keep the interest). If you don't hear back in 7 years or so, you have a growing fund for kid's college or retirement.
JMO