Welcome to FDMB
As Liz said it is overwhelming at first, but it gets easier by the day, before a cat is diagnosed with Feline Diabetes, it is extremely possible that as much as we look out for them, cats do hide illnesses quite well, once on insulin and the right diet and home testing, he will be gaining weight, it takes a few weeks to begin to see results, Corky was diagnosed 4 1/2 years ago, he was 10, indoor cat from 16.5lbs to 8.5lbs at time of diagnosed, so do not be discouraged, normally an inside cat can live to be in its 20+
We would like to know what insulin is your cat using, what is his diet? diabetic cats need to have a diet of wet can or raw foods between 0-10% carbs. Most members use Fancy Feast Pates between 0-10% most feed up to 5% (food list below, the third column contains the carb%, we are looking for Dry Matter Carbs, calculator below) if you are feeding kibbles (dry food), the transition has to be a slow one since changing carb content can alter the levels to low numbers, also feeding 3-4 meals or snack during the day plus the two main meals before shot, the CGM is a good choice, but it is also good to have a manual meter handy, since the CGM tends to read low numbers lower than normal, so you want to double check, as well as that the sensor may fall off or malfunctions you have a backup, a non-diabetic cats' glucose lever is between 50-120 most members use the ReliOn human meter and strips (Walmart) or any human meter, we base our numbers on human meter, no need for a pet meter, too expensive and so are the strips. Please continue to post any concern of yours is not a small one.
Also you should create a HYPO KIT containing some medium carb foods between 11-15% carbs, some high carbs between 16-24% carbs, some KARO syrup or honey, this in case his glucose levels run low. We can guide you along this process, also read our sticky notes in our main Forum, they contain much valuable information
Info - Dr. Pierson - new Food Chart
Carb Calculator - Felinediabetes.com