“And if you need a rocket engine, you’ll be the envy of the whole gang~"
This is the line that Phineas sings for this part. Before this picture, he was reaching up to press something in the front, probably to start the rockets.
As you can see, Jeremy stops him by simply holding his hand up to tell him "No, it’s okay,” and Phineas looks horribly disappointed.
To me, that speaks wonders. It means that Phineas wants to show everything that he had done to the car - as he said, “We worked really hard just to trick it out for you!” And when he wasn’t able to, a little part of him went, “Aw, but… but… We worked hard… And it’s really cool… Just for a few seconds? Please?" I mean, he quickly got over it, but still. This is the boy who makes anything and everything purely because he’s bored, or because a friend asked a favor. When he does something, it is never shabby, or under-done, it’s always over-the-top, amazing, and awe-inspiring, and dang it, when he makes something he wants to show off a little! Only a little - he’s actually very humble, if you think about it.
Just think of all the times he’s went on a quick, salesman-like pitch to explain every aspect of his and Ferb’s projects - he’s used to telling about everything that he did, and physically showing it to people so that they know he actually did it, and that he wasn’t lying.
But then when he’s not able to, he doesn’t overreact. He doesn’t go on a tantrum. He doesn’t complain. He just accepts it. And that shows how much of a truly honest and wholesome guy Phineas is.
(Yes I realize he wouldn’t have been able to have a tantrum because it was during a song, but if that was all it was, why even include him looking disappointed?)
This also speaks a lot about Jeremy - he knows Phineas tendencies and accepts, even encourages them. Just like he knows, accepts, and sometimes encourages Candace’s craziness, because he knows that that is a part of who they are.
Jeremy is just an extremely accepting guy, and a wonderful person to have on the show. Candace, you’re a lucky gal ;)