Welcome, and bless you for taking a diabetic kitty!
Yes, you can do this. The learning curve is steep and it can feel really overwhelming at first, but once you get the routine down, you'll be fine!
Starting point:
1. Get a cheap human glucometer from any pharmacy that's near you and convenient. Most folks around here use one of the relion meters from WalMart. I use this one
https://www.adwdiabetes.com/product/8195/agamatrix-presto-pro-blood-glucose-meter-kit-and-strips because I can just order online (no WalMart near me) and it's cheap.
2. Food: definitely get him off the DM food. It's too high in carbs for a diabetic. Fancy Feast Classics and Friskies Pate are low carbs and fine for feeding him (and the rest of your kitties). Friskies is the cheaper option. Be careful that you are getting the right varieties (Classics or Pate) because the other versions of those foods are too high in carbs.
Food is one of the most important things in managing diabetes. I really can't emphasize it enough. If he's getting into the other cats' food, or a dog's food, or high carbs treats (even one!), it will make a difference. That's why if there is any way you can switch over the other kitties so that there is absolutely no way he's getting into higher carb food, it will be best. And it really is okay to leave out wet food. My cats free feed on canned or raw cat foods all day long with no problem. I understand that may not be an option depending on your rescue situation, but thought I'd throw it out there in case there is any way you can make it work.
Make sure you get the glucose meter
before you make the food switch, as getting him off that high carb food may make a big difference in his insulin needs, so you'll want to be monitoring him before you make the change.
We can definitely help you interpret the numbers you get and figure out dosing. And yes, it does change over time, so daily monitoring is important. Again, that can sound overwhelming at first, but it's really not difficult once you've established the routine.
At the top of the Prozinc forum, there are threads with a yellow "sticky" icon at the beginning. Those are essential reading to learn the overview of how all of this works. Again, it's a lot all at once, but make note of your questions as you read through, and ask as many and as often as you need. We are all here to help!