If you have an old silcone spatula they work really good for opening their mouths. The rubber is soft so it doesn't cause gum trauma. Another added advantage is that it supresses the tounge and gets you in past the trachea. This always made me feel better about giving oral meds, less chance of them inhaling it.
If he starts eating on his own again, try injecting the meds into feeders. They seem to appreciate this alot more, and the meds wont get bound up in the mucus. I just measure out the meds into the oral dropper they give then suck it out with a syringe, as mine are not the same size. Make sure you leave that lil tip, on the oral dripper not the syringe, full of meds or you will be giving too much. The crickets generally live for about 3-4 minutes after you inject them and can hold quite a bit of fluid. If I noticed her hydration slipping, I would just inject some extra water into some feeders. I was doing this mainly to avoid any undue stress on the kidneys.
If you have a calander thats blank start taking notes. Days when hes doin well, or bad, his weights, how much he eats, if you needed to help him, general coloration. This way you can track his progress. As he is still young he should still be growing or maintaining a constant weight. I found it just helps to track trends. I learned things that I never would have without it. Such as how injections affect my girl, weight loss even given increased food intake. I plan on continuing this and do it with all my herps now.
As a side note, my female's uri was caused by citrobacter freundii. It was cultured after a tracheal wash, and found to be susceptable to baytril, fortaz and sulftra trim. All three drugs she had been on prior to the wash being done and the infection was not cured. The baytril and sulfa trim were given orally and the fortaz via IM injections. In the end it was chloramphenical that eventually relieved the symptoms *knocks on wood*. I gave this to her injected into crickets and within days it cleared. This after two months of drugs that should have done the same thing.
I've done alot of reading into URI's and causes of them in the last few months. If you have any questions feel free to message me, I would be glad to help. I hope he starts to turn around for you in the next few days!