For now, until you can get to the vet, i would say Fancy Feast classic varieties and i THINK the grilled ones are ok. Nothing with gravy. No soufle type ones either. Are you in the US?
How much does Sam weigh? How much do you feed him right now? One, two, four cans a day? I would put down half a can of FF 3-4 times a day if possible.
Is the iams senior dry or wet food? If dry, and if he is having issues not having it anymore, you can still give him small amounts of this i would say. At least before the vet visit and a proper reading of his blood glucose numbers.
One unit a day of Lantus is what my Dweezil is on too, for now. What number was his blood glucose at diagnosis?
Try not to worry too much! This is all so new and a big learning curve. You and Sam will get there bit by bit.