Sounds to me like you're doing the right stuff re: amount & type of food Buddy's getting ... Is great that you're feeding all 3 of your cats the same low-carb canned, too! (Yes,
definitely withhold all food in that 2-hour window before Buddy's AMPS/PMPS time.)
Buddy's weight history sounds so much like Bat-Bat's! She was 18 lbs in December 2012; when dx'd in August 2013, she had dropped to just a bit above 9 lbs. She'd been eating mostly so-called "healthy" dry kibble & only a little canned (didn't know squat about low-carbs back then). Silly me, I thought the weight loss was due to cutting her dry rations back a bit. (Wrong!) And as she'd had a UTI back in 2009, I perceived all that extra water-drinking & peeing she was doing as a "good" thing. (Wrong!) Had
zero knowledge about the symptoms of onset of diabetes! So was totally blown away when we got that diagnosis ... Today, now that she's well-regulated, she holds weight pretty steady at around 11 lbs.
Sue was an
enormous help to me in tweaking ProZinc doses for Bat-Bat earlier this year - you're in the best of hands!

Can't say enough good things about her wisdom. (Got me through some scary moments!)
Will be rooting for you and Buddy, Carole (& will be taking peeks at his SS). Will look forward to your posts on how he progresses!

- Robin