My Mum found my Dad through the internet (after I asked her to find him when I was 12). I can't remember which exact sites she used - but she searched for his full name, birth date and the state he lived in. There were about 200 results of men with his name but she wrote to every single one of them asking if they were him. Eventually the right one wrote back! But, I know it took a while (about a year, maybe a bit more) but she just kept trying and eventually found him.
Anyway, I hope that your fiance meets his Dad - it will take time and patience but hopefully you'll get there in the end :)