I hate seeing him struggle to be nice to his brother when I know that he wants to be. He is frustrated and doesn't know how to verbalize it.
Today, we started using a chart to help him to practice being 'nice' to Daniel. Each time he is kind to him in some way, he gets a star on the chart. When he gets 10 stars, he gets to watch a movie. It's not the most natural scenario, but it has already made him more aware of what he is doing throughout the day. Tonight, he needed three more stars and he kept coming up with new ways to be nice to Daniel so he could earn those last stars.