My youngest just potty trained herself about 30 minutes after we stepped onboard the Disney Fantasy this past spring. Totally not what we had in mind, but we rolled with it and that was that, she was/is trained.
We had tried introducing the potty months before the trip (we did the 3/31 inaugural sailing) because she can swim and we knew she'd want to be in the pools with her older siblings. Kid acted like the toilet seat was on fire anytime we sat her on it. Both the potty chair and the real toilet. Seriously. Wanted nothing to do with it and was content with her diapers. We told her often though that she had to use the potty to swim on the Mickey boat.
Well, apparently she was listening cause we stepped foot on the ship, she took off her diaper as soon as we walked in the room, said "I pee potty. No more bie-pers!" And that was that. Suddenly she went from the toilet being on fire to the diapers being on fire and refused to wear them, absolutely refused. We went to the gift shop, bought her underwear, had maybe 3 accidents all week, including our day in the MK and on the 5 hour flight home. And she was not, and still is not, a kid who will sit on the toilet simply cause you ask her to. Has to be on her terms only. THAT part was hard, trusting that she'd tell me she had to go, and with enough time, but she did.
Totally wasn't in the plan, but I thought "This is the window. If I set her back and push the diaper, who knows when she'll want to train." She was 2 years, 2 months at the time.
So, clearly, my advice, roll with it. If you get there and he's having one accident after another, I'd pull out the diapers/pullups. But if it seems to go okay, just go with his lead.