Hi Amanda,
You are in for a lot of fun at the Caribbean Beach Resort!
It is true that there are no microwaves in the rooms. However, I noticed a microwave in the food court when I was there in December - you would be able to use this to warm up your son's bottles.
This is a large resort, comprised of six islands (groups of buildings). The food court is at an area called 'Old Port Royal Centertown'. You may want to request a 'preferred' room in either the Martinique or Trinidad North buildings (both are right beside Old Port Royal Centertown). This will mean you will have just a short walk to the food court whenever you need to use the microwave.
As an alternative, each room does have a coffee maker which could make some piping hot water to use as a 'water bath' to warm the bottle in.
I hope this helps Amanda. Walt Disney World has many amenities to make it easier to travel with a little one. Check out
this link for more information on travelling with little ones at the theme parks. Have a truly magical vacation!