Okay, this is fine, and it's normal.
Yes, it IS like a light-switch. They wake, tantrum, then after they are back to normal. No big deal.
Sometimes, they are just still tired, or not fully awake yet....and they will tantrum when WE interfere in their "need" to pace themselves.
My daughter is like that too... she is just grumpy when she wakes from naps, even though she naps for 2 hours and wakes on her own.
This is just HER pacing, and HER trying to get awake and HER manner in which she is getting equilibrium again. When she is like this, I just let her be. She needs to "wake" in her own time... and not be rushed. She also does not like to be fiddled with or interacted with when she JUST wakes up... no talking, no engagement, no nothing. She HATES when she is "nagged" upon waking. Hey, I remember as a child, just getting so irritated when my own Mom would just start yakking away and fussing over me after I woke up. UGH! Same thing.
Just let your son be. You do NOT have to do anything, when he wakes. Let HIM do it. It's fine.
There is no reason a child HAS TO be picked up right away after a nap, and no reason they HAVE TO interact right away. When a child is just waking up, sure, they look disorientated... its fine. But when they FULLY wake up and get their wits about them again, it's fine.
My daughter would often tantrum after waking... it's just her and the way she is. So WE had to learn to just let her be. There is no reason we have to "shush" a tantrum when they are just waking. They are simply not fully awake yet. Even as adults... for me at least, I don't like to be "rushed" or pounced upon until I am fully woken up or "ready" for the day.
Sure, you can be in the same room, so he knows you are there....but no need to interact or even look at him when he is just waking. My daughter, she just likes to know that I am there...but if I even make eye contact or try to talk to her, she will tantrum.... she is NOT ready to engage yet. So. I just am there...in the room... or around... and I just let her be. When she is ready, she comes to me and then she is ready for the world again.
It's no big deal. I think as parents, we often think we HAVE TO DO something whenever a child "tantrums." BUT... you have to gauge it. Doing NOTHING in response to these "waking tantrums" sometimes is just as effective. Sometimes kids just want to be left alone, period. There is nothing wrong with that. Kids will mostly tantrum when we interfere in their pacing, especially when waking up. Some people are just not good "waker-uppers." No problem. My sister was like that too, when she was a child.
Any tantrum... either warrants nipping it in the bud...or, just letting the child vent. I know that sometimes, my girl just needs to get her "yah-yah's" out...then she feels better. It's fine.
Just let your son be, when he wakes. You already know what he does NOT like when he wakes... so just comply. He will get back to the "world" after he wakes... and sometimes, they even go back to sleep on their own, after that. My daughter does... its a "cycle" she goes through.
My son, when he wakes from naps sometimes, I don't speak, I go in the room, look at him and I can immediately tell what mood he is in... sometimes he does NOT want to be hassled or interacted with right away either. So, I just open the door, leave him in the crib, and then walk out... when HE is ready to engage, then he calls me. If I "rush" him out of his sleep or nap...this is when he gets upset and does not wake well. Let your son have HIS timing and style of waking.
Good luck,
Susan