I can't comment on the diabetes side as I fortunately have no experience of that. However regarding a bowl to slow his feeding down, I bought on-line a bowl that had moulded in to the base humps/ridges which certainly slowed one of mine down. I only used it for a month which seemed to do the trick and he then progressed onto a larger bowl.

The same puppy also went through a phase of every time he passed his water bowl, he'd have a drink. Again, he just grew out of it! I hope you get your pup sorted soon, as I know how worrying it is when there are problems that we don't understand and can't actually ask the dog about!!