Word of caution, some ports in Mexico and Canada do not even allow you to bring prepackaged goods or restrict what prepackaged goods you can bring based on ingredients and where they come from and they can be fines and/or jail time if you do bring them. I seriously wouldn't bring ANY food into Mexican or Canadian ports.It's agricultural importation laws that dictate foods coming into a foreign port. Even if the DCL CMs don't say anything (they usually catch food going out), there are often officers and sometimes dogs patrolling piers and the general port area. No doctor's note. There is nothing DCL can do to waive the laws.
Pre-packaged, factory-sealed, non-perishables is the general rule. Will he eat any snack type food? Bags of Chex mix, or little boxes of cereal? The little cereal boxes are available at Cabanas in the morning, and you can grab a few extra to toss into your excursion bag.
How long is the excursion? Could you make sure he eats a really good breakfast and then be back for a late lunch? Since this is his first cruise and (I'm assuming) first time in a foreign country, I would go with short excursions rather than full-day. If he ends up having trouble coping, you would be stuck waiting for transportation to get you back to the ship.
Enjoy your cruise!
That being said, I am truly amazed that Disney is still sailing to Mexico, even the ports they go to don't really feel safe anymore and with the current political climate, I have to imagine that has only gotten worse since I last went. I seriously would not be getting off the ship in Mexico.
I have been reading a bit about some new issues with cruising lately and here is a new wrinkle that has started occurring on cruises to Mexico on all cruise lines recently:
If you have any sort of medication, make sure it is allowed in Mexico, even if you aren't going to get off the ship. Authorities can now demand access to your cabin and if they find drugs (prescription or otherwise) that are not allowed in Mexico, they can and will arrest or fine you. Now what are the chances they will do this to you? I don't know, but they can. Additionally, legitimate prescription medication needs to be in their original bottles with the note of what the pills should look like from the pharmacy and you need a copy of your prescription.
So be very cautious when it comes to cruises to Mexico, especially with any medical needs and if you do get off the ship, do not wear clothes that make you look like a tourist or that you have a lot of money, that will also make you a target.