When mine was a hatchling she made a clicking/chirping sound when annoyed, rather than a full-blown hiss. I freaked out about RI, too, until I figured out the cause and effect i.e. being handled, in shed, etc. Just her way of communicating, I guess.
I'm pretty sure with infection it would be all the time, not intermittent. See if you can spy and listen to his normal breathing when he's not being handled, and if it gets worse/ more frequent. :-)
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