Due to that ridiculous bug where Loyal will disappear after completing Volare!, and spending the last 2 days attempting to force him to respawn near Black Mountain, I finally gave up on ever getting his special upgrade, nor getting the enjoyment of seeing one of my favorite companions not receive his happy ending. Nothing more to say other than that. Wish this quest would finally receive an update, already! Till then, poor Raul is "dead" to me (put a Raider mask on him, so I don't have to be reminded of his sad story.)

Thats a shame

Though did you check Camp Forlorn Hope? He might turn up there with the rest of 1st Recon after you complete Three Card Bounty or kill either Nephi/Cook-Cook and report back to Dharti

He might be in his house. Also, you can't trigger Rauls quest after talking to him without Raul there for the first time.
You have to make sure Raul is there when you talk to Loyal for the first time. The other two Andy and the sniper, can be done by talking to them after the first time.

Yes, Papafern, I'm after Loyal, not Corporal Sterling, thanks for the help, though. I was able to talk to both Sterling & Andy, though Loyal is completely missing. I'm 100% he's not in his house, Zx. I've gone through all of Nellis, several times, going from the arrays, to both hangars, I even managed to find a way to jump inside the barrier around the Pearl aircraft that was being worked on to see if he was inside (he was not.)

Then he's gone probabely. You can do Rauls quest next time.

My bad
Have you tried Black Mountain? Or in a straight line search from there to Nelis?
I had a problem before with Loyal and the quest you're doing, where Loyal after Volare! and when Old Time Ghoul(?) would just vanish off the face of Vegas, at the same time I thought I'd ignore this and go to Hidden Valley via Black Mountain and when I got to the top of the hill Loyal was just stand there with his normal dialogue so I engaged him in dialogue and after it he went on his merry way back to Nelis, I've had this happen with one other NPC too.

NP, and yes, I actually have fast traveled to Black Mountain, set my map marker on the Nellis Hangars, and literally ran all the way halfway across the map to them, still no Loyal. Oh and the name of the quest is Old School Ghoul (I remember it because they rhymed ghoul & school.) I also tried looking for him in Hidden Valley...no luck. I'm just holding out that the game will receive a patch that will someday fix this frustrating bug.

No patch.
I assume your on Xbox or PS3 so you'd be unable to do the command that teleports NPCs to your location?
He might have been at Black Mountain and died because on the route the NPC might take he should, possible, come by a Cazadore nest. Maybe try Jacobstown....I don't know why though, maybe those Super Mutants stole him?