Do you have a confirmed diagnosis of stomatitis - ie, does the inflammation extend beyond the gumline and into the throat and/or cheeks? If so, unfortunately you really are not going to see oral improvement without at least a partial mouth extraction (all the teeth behind the canines and often times the incisors), if not a full mouth extraction. Unfortunately, extracting teeth here and there doesn't work with resolving or improving true stomatitis. As daunting as these procedures sound, kitties do really well with them (much better than us humans). They are so relieved to have the source of pain gone that once they heal up, they truly thrive! And, fortunately they don't have vanity issues we humans have about having their teeth extracted. As odd as it sounds, they really don't need their teeth!
However, if it's not truly stomatitis, but rather gingivitis or periodontitis (the inflammation is restricted to the gumline and does not extend into the throat and/or cheeks), then extracting just the bad teeth should help improve things - at least as far as oral discomfort goes.
Another thing to be sure of as well, whether the extractions are for stomatitis or other oral issues, is that ALL of the tooth has been removed - that no bits of root have been left behind. This is ensured by taking x-rays after the extractions are done (while the kitty is still under anesthesia). If anything at all is left behind, it will continue to cause problems, especially with stomatitis kitties.
Some stomatitis kitties do need the support of prednisolone, and while this is not ideal, especially for diabetics, it can make a big difference. If things head this way, insulin does can be worked around the steroid. I would only consider steroids if inflammation and pain continue despite a full or partial mouth extraction that has had confirmation that all roots have been removed. I would not jump to steroids if he still has most of his teeth (if he is definitively diagnosed with stomatitis).
At this point, is your only concern that his BGs are staying the same as they were before the extractions? Or is he still exhibiting oral discomfort? Have you had a follow-up appointment with the vet who did the extractions? Personally I would not treat with the AB solely based on his BGs not changing after the dental. I would confirm with an in-person exam at the vet that there is an infection present before doing any more ABs; otherwise you could be opening yourself up to AB resistance, which could cause big problems down the road. Unfortunately some diabetic kitties' BGs do remain the same even after dentals, but even if that is the case, hopefully these kitties feel better after having bad teeth removed.
