Hi there
unfortunately, you should be testing his BG.
Have a look at these home testing links, honestly, it does get easier, takes a little practice and patience at first, but home testing is a valuable tool when treating diabetes.
Ask yourself this: if you had to give yourself or your child a shot of insulin, would you check yur BG first before shooting to know if it were safe?
Home Testing Links
If you're in the U.S - your best and least expensive bet may be the Relion meter from Walmart, strips of 50 are about $20, the meter itself is abut $9-$12.
What kind of tooth issues? You'll prob want to have that taken care of as it can affect BG levels, once tooth issues are good to go - you'd be surprised at the way some kitties drop in BG. Again another reason to home test before shooting.
What kind of insulin are you on?
What diet is your kitty on?
If he's not eating well, could be the teeth issues, some suggest baby food (no onion/garlic thou) parmassean cheese, tuna + tuna juice.
Others will be along to help you out, check back often! You've found the best site for feline diabetes!