No need to wait for Isla Mujeres if you are wanting to do discovery dives. Most dive centers with a pool will do "Scuba Experience" type dives so his first exposure to diving isn't as intimidating. A couple of pool sessions may make him want to get OW certified and let the real fun begin.
+1. Does your dad already swim well enough? Then, the next step is to introduce him to mask and snorkel and weight belt--in the shallow part of a local pool. Then add fins and easy surface swimming (in mask + fins + snorkel + weight belt. This can be done slowly, a day/week at a time, well before you leave for any planned vacation. This is enough for him (and you) to get a sense of if he actually wants to scuba dive.
Some people will want to leave (skin dive below) the surface. Some people won't want to, may never want to. A friend's wife, a non-diver, went with us on a sailing and diving trip to BVI. She had decided that she would snorkel above us as we dove to moderate recreational depths whenever we moored the boat. By the end of our trip, she had decided that she would get certified ASAP, which she did upon returning home.
Your dad (and you) will know from the pool sessions whether he will want to continue to the discover scuba step. In the pool.
Absolutely have your dad go through a diving physical--that is, a physical conducted by a physician who understands what's necessary to dive safely. Your dad is either okay to dive, or he isn't. The physical will tell.
John Glenn went through an extensive/comprehensive physical before he was allowed to go on his last space flight after he left the U.S. Senate. It only makes sense.
My dad will turn 90 in a couple of months. He is still chopping down trees and splitting firewood. Tough Army veteran. Swims, too. But he wouldn't try my scuba gear when my daughters were being introduced to scuba in my parents' backyard pool a few years ago! Absolutely not! (He'll wear a mask, though, when he's scrubbing the pool.)
Good luck,
rx7diver