Injections work better with snakes, relating to their slow metabolism I believe. You really need to get him to a vet. Most of the better medications for treating RI are only available with a veterinary prescription.
However, I understand that in some areas reptile vets don't exist. I find it odd a vet would refuse to see a sick animal of a species they normally treat because they are booked up, though. You might try calling back and making sure the person you are speaking too understands that the animal is sick and getting worse and may die or at least have the infection so advanced it is untreatable if you have to wait more than 2 weeks. If they still won't see your snake, then ask them for referrals to other nearby vets that treat reptiles. If it is the same vet that prescribed the oral medication, you might want to look elsewhere anyway. I sometimes wonder if the vets that prescribe oral medications for snakes do so because they themselves don't feel confident enough to teach an owner how to give the injections, or perhaps don't even know enough about snake anatomy to be able to do it themselves.
If that doesn't work, you might google for information on treating RIs with F10SC and/or with Tylan. F10SC is an antiseptic solution that appears to be able to cure minor RIs when used in a nebulizer and might at least slow the progress until you get to a vet. Tylan is an antibiotic that is available OTC for use in cattle or something like that, but you can get it without a prescription. I think I may have heard of at least one other antibiotic that is available OTC that might be useful in treating RIs, but I can't remember which one right now.
Here are links to a couple of threads on this site discussing the use of these:
http://ball-pythons.net/forums/showt...-from-Home-DIY
ball-pythons.net/forums/showthread.php?214205-Ball-Python-RI-gone!-Tazicef-F10SC-fogging-amp-the-cure-TYLAN
Please note, I really recommend you get the snake to a vet ASAP, but if you just can't, some treatment is better than no treatment. Don't think that just because you are giving your snake antibiotics (if you go with the Tylan) that it will get better, because not all infections will respond to all antibiotics.
Good luck!