Hello, welcome to the forums! They do worry you, don't they? LOl..
No, it doesn't sound at all like an Upper Respiratory Infection to me. Usually with a URI they will cough, wheeze when they breath, have a "crackling" sound when they breath, and have a ton of excess mucous in their mouth so that you see thick strings of it when they open their mouth. Also, he's eating normally, so no loss in appetite and no lethargy, so no, he's not sick.
They open their mouths as a way to regulate their body temps. It doesn't mean that his temperature zones inside his tank are too hot, it doesn't mean anything except he's regulating his body temperature. And they also do puff-up their beards for a number of different reasons, but that was most likely related to the mouth gaping to regulate body temperature. Also, they do often store food in their beards when they eat, and you won't know it, then you suddenly see them puff it out, open their mouths, and then swallow, and suddenly their beard is smaller than it was, lol. So it could have been that too. They also puff up their beards when they are going through a shed, to loosen the skin, just as an FYI. They do this for a ton of reasons, but none of which have anything to do with a URI. He's fine.
Just another FYI, humidity inside his tank is not a big deal at all either, as it's not with Bearded Dragons at all. They are completely different in this aspect than any of the tropical reptiles, as humidity doesn't effect them at all unless it's out of control. Most experienced Beardie owners, including myself, don't even look at the humidity at all really. They can live in a humidity of 10% or a humidity of 70%, and anywhere in between. So don't worry about that. What you do have to worry about and what does have to be very specific and exact are the 3 temperature zones inside his tank, and his
UVB light. Desert reptiles rely upon proper temps and adequate, strong UVB/UVA lighting pretty much for every bodily process and their lives literally depend upon them...but humidity is pretty much a non-factor...
As far as a respiratory infection goes, what does cause one is if he gets water into his lungs, by say putting his head under water during a
bath. That's probably the #1 cause of URi's in Beardies. Just a pro-tip, lol...