I read this book in approx 1974 (latest cut off date), it was more probably between 1968 and 1973. 
The book is about a kid about age 10 who loved baseball, as I did, and he liked to play catch. And meanwhile he meets a new kid about his same age and they become friends, build a bond, which eventually leads to the two playing catch. The baseball becomes a symbol of their friendship. The plot twist is that this friend is actually an alien, whose parents are aliens. I don’t recall much more of the plot. I think the alien kid is waiting until his parents repair their spaceship so they can continue their galactic journey. So when that is accomplished, the alien dad, mom, and son (the alien kid who had befriended the earthling kid) are in their spaceship ready to continue their journey, and they reach a point when they cross it the alien boy will forget everything about his time on Earth. When they do this, one of the alien parents asks the kid, what is that? And they are referring to a baseball which the earth kid had gifted him as a memory/souvenir of their friendship. Only, being past the “point of no return,” the alien boy doesn’t recognize or know what it is, and, being rather meaningless, he or the family eject it from the spaceship (I think throw it through the window, but I could be wrong). Their journey continues. That is the basic plot. I was not happy with the ending!