We did the nursery with my daughter last cruise. It was amazing!!!!!
The CMs are perfect with them. I think that is a great idea with the dinners.
We had issues with getting milk, but they have rectified it with having milk at the drink stations now.
Bring a lunch cooler for your son and pack milk and whatever else he will need into it for dinner - they will serve him. You tag the bag ith your cabin number and his name and bring it with you.
The one thing that drove me nuts was that the nursery was closed from 1-2 or 2-3, I can't remember exactly when, but what I do remember was that it was when we really wanted to add hours.
You will have no issues adding hours when you're on board. Don't panic. Port days are a little rough trying to add hours onto during the day, but nighttime is not bad and you should be able to add hours when you need them at that time.