My mom recently had a cat who had hepatic lipidosis (aka "fatty liver"), and he had an e-tube, too. He wasn't diabetic, but I'm guessing your concern is more how to take care of Boo when he is released from the hospital? Two of my cats also had e-tubes, as well. It's intimidating at first but after you get the hang of it, it just becomes routine! So obviously, the most important medicine for a fatty liver kitty is nutrition. In that sense, having the tube in place is actually going to make this easier for you because Boo probably doesn't want to eat much, if anything, right now. That's normal. With tube feeding you are in charge of what you cat is getting, so follow your vet's instructions. Usually kitties do better with smaller meals throughout the day instead of just 2 large ones. Tube feeding is very easy if you haven't done it before. Canned food (usually something high-calorie or for critical care, like Science Diet A/D) is basically pureed into a mealy-like texture, sucked up with a syringe, and plunged down in the tube slowly. Then you flush the tube with 3-5ml of water afterwards, cap the tube, and you're done! And if your cat is on any medication, that can usually be crushed and inserted into the tube as well (which is really sort of a bonus - instead of wrangling w/a pill and a feisty cat who does not want to open their mouth you can just push the meds down the tube). You may want to ask about a liver supplement such as Denamarin, as well. It doesn't require a script (you can buy it off Amazon or online pet supply stores) but includes ingredients such as SAM-e and milk thistle which is supposed to help the liver heal. The pills are really huge though and I'm not sure if they can be broken up and flushed down the tube, so it doesn't hurt to ask.
Oh, and one more tip: I'd recommend purchasing a
Kitty Kollar to keep the tube out of the way. It's a lot less messier than dealing with the bandages and gauze that some vets use to secure the tube around the cat's neck. Plus, they are very fashionable and launder well. I know it was important for my cats to feel chic when they were recovering from their illnesses (haha!).

I'm surprised how many vets don't know about them, but I'm always happy to educate!

They are pretty inexpensive, too.
I hope Boo starts feeling better soon. The earlier he gets treated the better the outcome, so it's good that he is being taken care of right now.