Very impressive that he can whistle two notes at once anyway

I would say Träumerei by Schumann.

Edit: Confirmed

I played it out to see if I could tell, but I don’t recognize it. Its not a particularly romantic or melodic thing tbh. Kinda mid.

Someone else already confirmed it, but here’s a page with some trivia like that:

https://en.wikibooks.org/wiki/Annotations_of_The_Complete_Peanuts/Print_version

It’s kind of hit-or-miss, but does list this one:

Charlie Brown is delivering the traditional whistle of appreciation for feminine beauty (though it usually has two notes: WEEEET-WOOO), and Patty takes offense. Schroeder, being more musical, delivers a mini-concert to Violet and gets to walk off with her. It is the melody to “Traumerei” from Schumann’s “Scenes from Childhood” for piano.

I’m terrible about reading notational music, but… isn’t that “Happy Birthday to you?”