My fiance is using "Somewhere over the rainbow" it was a song that his mom used to sing to him when he was little. Maybe ask your FH if there are any songs that were special to him and his mom? I think tempo doesn't matter so much as whether or not its meaningful to both of them? And if he does have a song like that with his mom I think it will be really special for her to have it come on without know about the song choice ahead of time.
Hey Sara, I am kind of in the same boat with my FH, guys can be so difficult sometimes lol! I think we have narrowed it down to "You Raise me Up" by Josh Groban, it seems pretty fitting and i think his mom will love it. I honestly just googled "Mother/Son wedding songs" and found a bunch!