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.
If I remember correctly, discovery dives are confined water only so boat diving won't be an issue but....
I travel with a group of older retired divers with various ailments that often require assistance...usually back related. Depending on the boat design, we will sometimes help a diver gear up on the swim platform, if equipped, or just gear up in the water. We have never had an issue doing this. When the dive is over, gear comes off in the water and is passed up. It's your dive. Be upfront with the dive op and make sure they can accommodate his needs. We have never had any issue at any tropical vacation location.
A little assistance to stabilize the diver up the ladder and to their seat is usually welcomed, especially if the boat is rocking.
Good advice on the medical form. I've seen some that require a doc's approval if you are over 55.
Good luck, safe travels,
Jay