I am insulin dependent, have been for 26 years now. I have had some hassles in Mexico but nothing to serious but did have an incedent that changed the way I do things.
In 2004 down there for a surf trip we got pulled over and the car was searched. I carry my insulin, tester, needles and everything in a travel bag that is always in my back pack. Well before he found that we found the cap, the bottom cap for the plunger of a needle on the floor of my car. He knew what it was, cuffed us all and sat us aside while they tore through the car. None of us could speak enough Spanish to explain what was up. They got to my bag, found my travel pack and realized what was up. By then an english speaking officer was there and told me to do a few things in the future.
1) Carry a medical trash bag for everything and make sure it all ends up in there and then keep this bag in a place where it can be seen eaaily when your stopped.
2) Wear a diabetic ID (Braclet, necklace) of some sort.
3) Only take down the needed supplies, do not carry to many needles, etc.
Since then I have never had an issue and have been pulled over a few times and searched. No biggie.
Also as an FYI for your guy - There is something in all the diet soda in Mexico that will raise your blood sugar a bit. If he drinks diet soda tell him to pack it down there.