If he is developing an RI there will be other signs to watch for so you can catch it before it gets too bad. I *thankfully* haven't ever had to deal with one before so maybe someone with more experiance can fill in any gaps I leave (or correct me )...

If you notice any weezing/popping type noise when he breaths, and if there is any fluid or mucus like substance in his mouth that you can see. Yawning more than usual could also be a sign, but yawning right after he ate is pretty normal. I would just keep an eye on him, and maybe bump up the temps a bit...what you have is good for the cool side (80-83) but you need a warm side as well at about 90-93.

And yes, RIs are fatal, but there are signs if your watching for them and if you take him to a vet when you notice them, they can give you antibiotics to fight it. Again, I would just keep an eye on him and I'm thinking you will see the signs if he has one.