I've had Jack (the kid) disappear on me once I arranged a meet and greet with the girl from the Crimson Caravan.
Luckily, I was on a quest which involved finding him, so a Quest marker indicated that he and his new girl friend were (somehow) wondering the wastes together. They started their little trip at Black Mountain and walked at a normal pace back to Nellis.
Once I had finished the part of the quest that he was required for his quest marker had disappeared, leaving me with a whole bunch of scrap metal I have no potential use for. (I wasn't into making things.)
Luckily, i'm on the PC so I used this console code to bring me to him:
player.moveto 000ff268
Assuming you're on the PC, that should work.
If you're looking for Loyal you can try typing in:
player.moveto 000ff26a
***
If you're on the 360/PS3, i'd suggest looking around Black Mountain (assuming you've been there) For some reason, Jack and Loyal have a tendency to spawn there if you've fixed Tabitha's Mr.Handy and free Raul.