"You'll Be In My Heart" I think is definitely the best and most appropriate Disney mother-son dance. You could also maybe use "You've Got A Friend In Me" from Toy Story, which is a great song. I did a mother-daughter dance with my mom, and we used "Forever Young" by Rod Stewart. I love it because it's a great song, and I feel it's a good choice for the occasion but not overly used. Just another suggestion!