Maniac and Grayson's Christmas is pretty great just as it is. But it gets even better when Grayson gives him a present:
Maniac tore it open—and gaped helplessly when he saw what it was. To anyone else, it was a ratty old scrap of leather, barely recognizable as a baseball glove, fit for the garbage can. But Maniac knew at once this was Grayson's the one he had played with all those years in the Minors. (31.12)
What's more, Maniac knows that Grayson has just passed on a little piece of himself: "Slowly, timidly, as though entering a shrine, the boy's fingers crept into it, flexed, curled the cracked leather, brought it back to shape, to life. He laid the new ball in the pal, pressed glove and ball together, and the glove remembered and gave way and made a pocket for the ball" (31.12). The ball just fits in the pocket, just like Maniac and Grayson just fit as father and son.
Even after Grayson dies and Maniac takes off on his own, he keeps the glove as physical proof of his family. And throughout the book, Maniac leaves one family behind as he moves to the next, but he protects his memories of Grayson by protecting the glove. When he finds the McNab kids playing around with glove, "using it as a football, punting it back and forth. Maniac exploded" (40.12-13).
For Maniac, that's it: this is when he realizes that he can't make a home with the McNabs. And it's a crucial step in figuring out what his real home looks like—with the Beales.