First of all - what are you feeding him? First thing I would suggest is that without changing his insulin dose, you switch him to a low-carb wet food diet. If he has been eating dry food, or a high-carb wet food, just the diet change by itself can make a big difference. You don't want to lower insulin and change to a low-carb diet simultaneously, though, because that could possibly send his blood glucose dangerously low.
Home testing is the best way to make sure he remains safe! Get yourself a human glucometer (I use ReliOn Prime from WalMart, as the strips are the least expensive), and test away. Try to get a curve at his current dose - test just before the shot, then every 2 hours for the next 12 hours.
You'll want to set up a spreadsheet to record these values as soon as possible - here is a link to the instructions:
http://www.felinediabetes.com/FDMB/threads/fdmb-spreadsheet-instructions.130337/
Then you can create a signature, which includes basic information about Motley, along with a link to your spreadsheet. This is so that members who respond to your questions can see the whole picture in order to give appropriate advice. I can't tell you whether you should or shouldn't increase Motley's dose, but if you collect data to see exactly what is happening with him, then it will be possible to help you make decisions regarding whether his dose should remain the same, or whether it needs to be changed.
What kind of insulin is he getting? Depending on the type, dosing decisions may be different. Lantus is the preferred choice for most cats; based on testimony of cat owners, it tends to give better results.
I'm sorry if you were looking for a simple answer and are feeling like we're just dumping tons of instructions at you, but diabetes can be tricky, every cat responds differently, and the best way to get help from the extremely knowledgeable and experienced members here is to provide as much data as possible.
Good luck with Motley - with proper diet, home BG monitoring, and appropriate insulin dosing, he can get back to good health quickly!