My guess is that he can't settle down enough and he's so used to waking up with some interaction with you, it's a totally ingrained habit. He needs more uninterrupted sleep to a) make him tolerable and b) have healthy brain development, so he has to learn to self-soothe. Yes he has some stuffiness but it doesn't stop him at 2 AM so it shouldn't stop him at 10. I'd say no drink of water, no talking (stern or otherwise), no snuggling, no music. White noise, maybe, if he's used to it. But the problem isn't that he wakes up - we all do. The problem is, he can't settle down on his own. If it were allergies related to the room or pillow or whatever, the same problem wouldn't occur like clockwork at other locations, would it?
Have you done any type of sleep training, Ferber, anything like that up to this point? Those techniques tell you to engage as little as possible, speak just a few words with a little back pat, then leave and let the child settle himself down. (No food, no snuggles, no lying down, no stressing out with stern warnings, etc.) Read up on them and see if you can agree enough to be consistent.
Allergies can be a factor, don't get me wrong. But there's nothing you can do with pharmaceuticals short of reducing symptoms and inducing drowsiness, but there can be a rebound afterwards, negative side effects (including dehydration, daytime grogginess, and dependency). I went through this my whole life and I was given everything, given nothing, the whole roller coaster you're on. For me, exhaustion actually makes allergies worse, so there's a vicious cycle that you can get into too. I finally got rid of all my symptoms using food science. That's an option for you but I think you'll still need the behavior modification piece to get him back to sleep - he really needs it. We did it with our son, and it was hell for about 3-4 nights, but then it solved the problem. He was younger than your son so there was no reasoning with him, we just did it on the advice of the pediatrician. He wasn't in the habit of getting up and engaging with us for as long as your son has been though, so it could take longer with you. I'd say to do it over a long weekend (such as Labor Day) so you at least have 3 days when you don't all have to get up and function. And, as you've noticed, YOU need your sleep too - I realize the 10 PM session doesn't mess with your schedule too much, but the 2 AM thing has to go, for everyone's sake!