Howdy Victoria! Opinions on this will vary but below are the preferences for my family.
We always try to find our parade spot an hour before the parade begins and you will find that others will do this as well.
Magic Kingdom: our favorite location is near the bridge to Liberty Square outside of Ye Olde Christmas Shoppe. You have a view of the castle, we can sometimes find some shade and the most important reason has to do with the handshakes you asked about for your son. There is a very small slow down of the parade due the bridge so the characters that are walking have a few extra seconds to add some magic to the day of several kiddos watching the parade. On multiple occasions, this location has given us a higher number of opportunities to be approached by a character and that has made the day of my 4 and 6 year old each time.
Epcot: there isn't a parade but, you might consider
Disney Phineas and Ferb: Agent P's World Showcase Adventure. You will become a secret agent and go undercover on this interactive quest around World Showcase. It's a lot of fun and will add some magic to your son's time at Epcot.
Disney's Hollywood Studios: we like to be on Hollywood Boulevard. It's a nice area to sit and enjoy some popcorn and cotton candy before the parade begins.
Animal Kingdom: here, we don't actually have a favorite spot. In this park, we view the parade based on the next attraction we are going to do so that we are where we need to be following the parade. We have enjoyed all of the locations and this parade has become a favorite of my little ones.
Thanks so much for your question and please let me know if you need any further information.