Celebrity has a fantastic kids program, your son will have a blast. We too enjoy traveling the world with our children, and a cruise ship makes it really easy to do so. Our kids are teens now, and Celebrity is their favorite cruise line.
For your own sake, it will be a much much better experience for everyone if he is potty trained and can stay with the other toddlers for a few hours.
Pro tip from a mom who’s youngest also had medical issues: you can potty train a 3 1/2 year old in a weekend. Roll up your rugs, and take the kid’s pants off. No diapers, no underwear, nothing. They won’t just go, they look for a corner or some out of the way spot to go. You need to be watching the whole time, and when you sense they need to go, put them on the potty. Repeat all weekend, praising lavishly any success. Reward success with m&ms. One m&m for a pee in the potty, six m&ms for a poo. He’ll catch on really quickly lol.