If it means that much to you you can hire an investigative agency. They will charge you a ton of money and the results are never certain. What if they say he has no ther children? Are you going to believe them?
"Out there", could be almost anywhere. Until you have more specific information as to who this child might be, where and when s/he may have been born and the child's current likely whereabouts, your efforts as described are likely to amount to little more than the proverbial "wild goose chase".