Did you remind the dental clinic that he's a diabetic and to confirm instructions with the vet? Sounds like you got the standard front desk response. I was always told I could feed 1/3 of breakfast before 6AM, and do half a dose of insulin. That is, once I'd told them to consult with the vet.
Why do you listen to the IM vet?

He does not seem to understand Lantus dosing. Has Blue ever had ketones? One skipped shot should not make a difference. It took a few days before you started insulin and he survived that. I would not give any extra insulin before your usual PMPS time, hopefully that'll be back home. You don't want to deal with multiple nadirs. The dental vet will give you instructions about feeding - listen to what they say instead. With no dose, or even half dose in the AM, he likely won't go hypo since he hasn't even seen green on this dose. But I'd still bring an assortment of foods, including HC. Ideally you should take a break half way through the trip home, test him, and see if he'll eat anything. When you do the PMPS test, you'll know if he's going up or going down. You can also ask the vet whether the effects of anaesthesia will change his BG. Likely not.
As a FYI, the times Neko had a dental, she ate like a champ afterwards. Even the surgery that required removing a bump from her jaw bone. Blue will come out well medicated. The question is whether he'll eat in the car. You can test that on the way down. Neko preferred the car not to be moving. Maybe at a gas fill up.