After viewing the case I fear it only depends on the motive of Mr. Utesch.
If Keller’s parents are divorced (which it sounds to be the case) it’s highly plausible that he couldn’t see his son and decided to take him. For most kidnap cases the child or person wouldn’t live for all this time but if it’s a parent it’s possible Keller might be alive and living with Micheal. Of course it depends on the motive, if Micheal was abusive to Mrs. I Utesch and they WERE divorced he might be more violent. This would mean less chance of Keller being alive.
Though another thing plausible is that Micheal wanted his son, took him in and illegally changed their names by moving towns and living with a new name. Meaning he might be anywhere.
I’d suggest asking the mother more questions about the relationship between Micheal and her. Possibly between Keller and Micheal and how was Micheal as a person.
These can help identify his motive and that can determine whether or not you’d think Keller’s dead.