What does the underlined sentence mean? Read the excerpt from "The Scarlet Ibis.”

When he was two, if you laid him on his stomach, he began to try to move himself, straining terribly. The doctor said that with his weak heart this strain would probably kill him, but it didn’t. Trembling, he’d push himself up, turning first red, then a soft purple, and finally collapse back onto the bed like an old worn-out doll. I can still see Mama watching him, her hand pressed tight across her mouth, her eyes wide and unblinking. But he learned to crawl (it was his third winter), and we brought him out of the front bedroom, putting him on the rug before the fireplace. For the first time he became one of us.