Laxatone is one common hairball remedy but it might be too high carb. Some people use plain Vaseline if the cat will lick it up - main ingredient in many hairball remedies. Does it look like he's trying to vomit or is it more like coughing?
It's unfortunate that he won't eat low carb grocery store food because they're a lot easier on your wallet. Did you get samples of the Young Again yet? Have you tried the canned Dr. Elsey's? I think you said he didn't care for the kibble.
Re feeding segregation: you might consider training your three to eat in separate rooms at scheduled meal times so they can each eat their own type of food. Then, take away all food bowls after the meal time and leave out some low carb kibble for all three to graze on. Yes, it will likely be hard and there'll be much protesting but as long as no one goes on a serious hunger strike it should work out over time. I'm sure you've thought about this type of thing but maybe dismissed it because you know your cats' habits, etc. Maybe give it some more thought? Remember - you thought you'd never be able to test Wilbur's BG and look how far you've come with that.