Not FP, but at 2 and 3 my daughter adored the boat ride in Mexico. We had to implement a 5 time in a row cut-off, or we would have rode it in circles all day. Luckily that line is usually short.
For World Showcase, we typically start at Mexico (as the boat ride is a must) which is the first country at the end which will be on your left as you are walking towards WS from Future World, and then we just work our way around from there. Part of it will depend on whether he has any interest in meeting any of the characters around WS. Most are Princesses and they tend to have long lines and take some planning. Donald (outside Mexico) is typically pretty easy to meet and Mary Poppins and Alice are also pretty easy to meet without long lines. DD was a Princess nut, so that involved some planning. If you aren't having to plan around character lines- we would just start at one end and go from country to country.