I think what the OP was saying was does it matter if you don't see Nemo before TT. !
I got the impression that this was just part of her query.
Is it a requirement? not at all. Does it help? Statistically, yes. Disney has made several attractions using characters that the audience doesn't know. Some of them work, some flop. (Splash is based on Song of the South. Guests under age 40 don't know it, yet the ride is very popular. PotC became wildly more popular after the PotC movies. Other WDW shows based on unknown characters have flopped. One was in Tomorrowland a few years ago. I forgot the name.)
I also kind of go back to the first sentence I posted...again I'm not trying to be snotty- but perhaps we don't have enough information. Maybe the OP didn't know about the upcoming Finding Dori sequel.
It also occurred to me maybe this boy doesn't know (m)any Disney characters. If that's the case, it probably will have some impact on his enjoyment. Odds are good, he will have some preference for characters he knows best. Today's kids don't pay much attention to the Pete's Dragon float, they don't know Pete.
One great things about Epcot- is that it only has a few FP attractions. There's really only three: TT, Soarin', and the new Norway ride (only while it is new). Most of the day, you can roam Epcot and do as you please. You don't need to decide in 6months in advance. In other words, probably the best approach is to let the boy decide what he wants to do, when he sees the attraction. My favorite part of the Crush show is the jellyfish tanks in the preshow area! but if he doesn't want to see the show, there are plenty of other options. The big aquarium is huge, plus the manatees, smaller tanks, and the electronic quizzes. The best approach is often to allow kids to explore the pavilion at their own pace.
Last, if the movies aren't an option- there's a ton of DISNEY books! There's aline of beautiful inexpensive anthologies. The library is alos likely to have a ton of books!