1. If you require that many sodas by all means get the soda card.
2. If there is something you are certain you want to do, I would book ahead. The shore excursions rarely sell out completely but I for one would hate to take that change on something I was sure I wanted to do anyways. If you have no idea what you would like to do, wait till you are on board and go to the presentation.
3. I don't know about the ship you are on, but most ships have some restaurants that charge a sur-charge. Of course there are always at least two that are free all the time. The regular dining room, and the buffet. Some ships have a premium private, reservation only dining room that charges a fee, and some have a chain hamburger shop, usually about $3 surcharge for all you can eat. Also coffee bars, and ben and Jerrys often cost extra.
4. If you can get a suit, or even just a jacket for your son that would be great. I think it is important for children to dress up every now and then. If he doesn't have one, or you can't afford it, a simple button up shirt and slacks should be just fine. My son is going to be wearing a sports coat and tie.
Sorry haven't done your ship or itinerary, so can't help with the rest.