Because there are so many UK expats in the Orlando area, many of the supermarkets there have begun to carry Marmite in their special "British foods" sections, but it isn't a sure thing that you will find it, because Americans won't touch the stuff. Your best bet is to carry it with you. Bring a factory-sealed new jar.
Blackcurrant juice is also something that is very rare in the States, and usually can only be found in the "British foods" section of a supermarket that has one.
Also, if he eats soft-boiled eggs, you should know that many US restaurants will balk at serving them, and especially serving them to a child. Some places will refuse to serve them to you, as the USDA has for years been advising restaurants that they are unsafe because of salmonella.