Shawn loved it when Gus was dangerous.
A little more spontaneous, adventurous, or did unsafe and perilous things - that wouldn't kill him - anything really, that wasn't the norm. It turned him on, Gus being so unrestrained, it was nice, he didn't know why he loved it, but he thought it was sexy.
So when Gus rolls in on that tiny motorcycle, whoa! He has to take a few breaths—and hope it isn't obvious—then poke a little fun with a hint to Gus about it later.
And then the prom comment. He had actually told his father he was taking Gus, Henry didn't think much of it because they went everywhere together and he thought they were taking girls. But then he and Gus went alone in their tuxes. Everyone in their class didn't care and they all had fun like always… Actually that Alice girl in his Chemistry class didn't seem too happy about his choice of a date, come to think about it. They came home alone, and his dad didn't talk to him for three weeks.
So yes, he was disappointed when they got banished from Dutch's little posse. That meant that he couldn't watch Gus be the daredevil anymore—or just have fun doing the stunts himself—but still every once in a while, he and Gus would go to an amusement park or something and they'd have fun.
Then they had some more fun at home.