Nice drop, but it is 50% in 3 hours. If you are going to leave before +6, I think I might give him a snack of lower carb before you leave.
You may want to do some research before you see the vet and have your info in hand: a steroid that might raise his levels less, an antibiotic that doesn't have any sugar (although maybe he can go off that). Maybe ask him about Miralax. It's something you could easily give every other day in a small amount.
Dana, it really is beginning to look like Riley's diabetes was caused by a perfect storm of an infection, a steroid and high carb food. If there is a way to alter those things, you might have a kitty in remission (anti jinx). You already made a huge difference changing his diet.