Whoa whoa whoa, how old is the child in question? I adore Princess Mononoke (and had the pleasure of traveling to Yakushima while living in Japan to go see the "Mononoke Forest"—highly highly highly recommended for anyone able), but it's a PG-13 movie that deserves the rating from the very first minutes when people start getting bloodily decapitated...
As said, pretty much every Ghibli film ever made is amazing, but be careful what age group it's aimed at. Try Spirited Away, Ponyo, Kiki's Delivery Service, or Nausicaa of the Valley of the Wind as more kid-friendly offerings. (Interestingly, you'll notice that they all have female leads, but boys will love them too, which is pretty awesome.) Try and get subtitled versions as 1. they're better and 2. more reading for your son.
Pixar's films are also incredible, maybe even more so for parents than the children! My absolute favorite is Finding Nemo (but for the love of all things good ignore Finding Dory, I have no idea how that movie went so terribly wrong) with The Incredibles a close second (and hopefully The Incredibles 2 will be a much better sequel, as I can't help but get my hopes up.)
Other shoutouts go to The Iron Giant, The Emperor's New Groove (super cheesy but it tickles my inner child funny bone), and Song of the Sea (which is just so lovely and enchanting).