@NessP,
Hi Vanessa,
The best person to talk to in cases like this is usually your mother. If she isn't alive, or she won't talk (and you are at the barest minimum entitled to know his genetic disease profile), talk to her siblings if there are any, and your grandparents if any of them are still alive.
You can also look a bit on Facebook or the like as it's possible that he joined (and be aware that for him to suddenly hear from you would be a major shock). Also try Google, as there could be some information, e. g. the purchase and sale of real estate, but also criminal records and death notices, are matters of public record.
Best of luck to you.
PS If you have some money to spend, hire a skip tracer; they should be in the phone book. Their job is to find people in order to serve subpoenas and arrest warrants so they know where to look but they are not cheap.