Thanks for all of your replies!

The more I look into it the more I feel that he probably doesn’t have IBD. I’ll be careful to watch, of course, over the next month of antibiotics and after that, and if I notice more symptoms then... that would suck.
But right now I really think it’s just an RI, he’s not showing any signs of neurological Issues and he’s still eating fine and behaving as usual. His RI doesn’t even seem to be particularly severe, I heard him make wheezes and have seen him yawn but he’s not keeping his head elevated, he doesn’t always make sounds, and he has had no discharge around the nose, no mouth rot, and excess mucus mostly just in the mouth and not in the trachea.
The only reason I haven’t done the blood test right away is because it’s fairly expensive for something that should make itself obvious fairly quickly. If he shows any more signs I may consider the blood test, but for now I’m just going to focus on treating the RI and keeping his normal routine.

Again, thank you, having more information from more perspectives is very reassuring and I appreciate it.