You'd be forgiven for thinking Horton as a massive elephant would be oblivious to anyone small compared to him, let alone infinitesimal. But the guy's size is matched only by his heart, he'll stop at nothing to protect the Whos, after all, "A person is a person, no matter how small".
Somehow they managed to sneak the line from Horton Hatches the Egg into the movie, as Horton explains that he promised the mayor to get whoville to a safe place. "And an elephant's faithful 100 percent."
The Whos finally believe in the mayor, cheering for the elephant that's going to help them.
When it looks like the world is going to end, the Mayor turns to Jojo and tells him that he couldn't ask for a better son. Then after the day is saved, Jojo tells his dad "You're one of the greats.", affirming a desire that his dad mentioned when their relationship was on the rocks, meaning that even though they weren't close at the time, his dad's happiness still mattered.
Rudy giving Horton the clover, and shortly after, Horton giving Kangaroo the cookie.
And then Kangaroo placing a small plant like an umbrella over the clover.
When Horton starts to sing "Can't Fight This Feeling," after all he did to help the Whoes against all odds, no one deserves to be called "A Candle in the window/On a cold dark winter's night" more than him.
The fact that everyone, including Kangaroo and Vlad, sang along in the Sudden Musical Ending, gives the viewers a good feeling that everything finally turned out alright for everyone.
Jojo joining in. After spending the entire film (and implicitly much of his life) saying nothing his voice really has a chance to shine here.