I flew with Neko from Vancouver to Denver, on our way to Fort Collins.
@Dyana used to fly to see family from the east to west coast and back, with her first diabetic cat.
I did not use gabapentin on Neko, just some Rescue Remedy and Feliway wipes in her carrier. Lots more hints on how to travel with a cat in here:
Travelling with Diabetic Cats
Are you planning on having Butch in the cabin with you? I can recommend the Sherpa carrier. At the time I flew, they were designed to fit under the seat in front of you.
Some notes not in that link are making sure you get documentation from your vet that he is diabetic and what meds are needed. Possibly also a list of up to date vaccinations. I had a small lunch sized container with a freezer pack and Neko's meds in it. As well as some food and HC. Which I stored in my backpack along with a make-do litter box. Get a small box, like a Fancy Feast sized cardboard box, wrapped in a garbage bag, and a very small bag of litter. Extremely important, store the litter in the original litter bag! I didn't, got pulled into secondary screening. Apparently it looked like something you can make bombs with.

When you stop at the airport you can put out the litter box and see if he wants to go before you board. And immediately after.
Good to hear his stuff will go on carryon. The cat in carrier counts as a carryon, so I could only carry the backpack in addition and checked my other luggage. Thankfully I had a couple days food for Neko, as well as her dishes in my backpack, cause my checked luggage took at extra day getting to my destination. Sigh! I had scoped out where the local pet store was closest to my hotel. Also make sure you know where the ER vet is at the destination, just in case.
Make sure he has a microchip and/or tattoo. I got a harness (Kitty Holster brand) and got a tag from Petsmart with her name, "diabetic" and my phone number on it. You will need to remove the cat from the carrier to go through screening and walk through the xray with them. Make sure the leash is well attached.
Depending on what his numbers are doing, you can skip or do half dose, or maybe just reduce a bit a couple days before you leave. The downside to high numbers is he may need to pee more. I was lucky that Neko's nadir was after we landed so I gave full dose.
And lastly, from experience, I can say that travelling with a cat in airports during US Thanksgiving week is not a good time to travel.

Neko was very quiet and mellow the whole time thankfully. It was me that was nervous. Best of luck with the trip. Let us know how it goes.