From personal experience... combination of altitude sickness and neck tension. I've had identical symptoms at same altitudes. Nothing to do with physical conditioning or hydration. In fact, when I was in my best shape, it was at it's worst. I could feel coming on as I drive up to my spot as a slight headache. Wake up the next day with a bigger headache but get through with aspirin. Next day... muey malo and no amount of aspirin helps. Probably makes it worse.
The whole thing got much better if I went up a couple days early and just ate and napped around camp. Actually, being by myself with nobody around to bounce ideas off of probably helped keep me down so I could acclimate. I'd just avoid physical exertion. No glassing or scouting. Just kicking back. Take a book or something. Frequent massaging on the upper backstap where it connects with the skull plate, along with numerous applications of Biofreeze (oh it helps) on the entire neck (into the hairline) and shoulders. Chances those two long muscles that run up each side of the spine and connect with the back of the skull and hard as rocks from tension. Gotta massage them pretty aggressively like Big Sally, the Swedish massage therapist is working you over good. Get some good meals, lots of rest and acclimate.
Here's one they don't tell you about in puking school. Massage your bare feet. Seriously. Your feet are connected to the entire nervous system. What the heck... you're sitting around anyway. Just like the back of the neck, work 'em good. Find the spots that hurt when you rub them, like between the toes, and massage them a lot. You can feel your face twitch as the tension goes away.
I noticed it was also much better if I could do an overnight trip or two for a couple days each at altitude. Doesn't have to be 10-K, but something quite a bit higher than home. Nothing big. Just drive up, pull off the road, set up a tent/hammock, a chair and chill. Next day, take a nice little walk and go home.
All this may sound dumb but to anyone like me who'd make a valiant effort just to stand up and walk down the trail due to nausea, you'd do just about anything to figure out how to make it stop.
Most guys just run 'n gun to their spot. Next day, hunt hard and they're fine. If the altitude is getting you though, it's days of misery. Hope your Bud gets to feeling better come opening day.