The Ghost of Christmas Present shows Scrooge the Cratchit family's present Christmas and predicts that future holiday celebrations will be minus Tiny Tim. The boy will die if the Cratchit family's life does not change in the future.
Scrooge is enchanted by Tiny Tim. The author tells us that he looks at him throughout this visit. He doesn't take his eyes off the small boy who loves life, even in his physically disabled condition. Scrooge is a miserable and lonely man who has everything compared to the Cratchit family, yet he is fascinated by Tiny Tim's simple joy.
"I see a vacant seat," replied the Ghost, "in the poor chimney-corner, and a crutch without an owner, carefully preserved. If these shadows remain unaltered by the Future, the child will die.'' (Dickens)
"No, no,'' said Scrooge. "Oh, no, kind Spirit! say he will be spared.'' (Dickens)
"If these shadows remain unaltered by the Future, none other of my race,'' returned the Ghost, "will find him here. What then? If he be like to die, he had better do it, and decrease the surplus population.'' (Dickens)
Scrooge hung his head to hear his own words quoted by the Spirit, and was overcome with penitence and grief. (Dickens)
In the above passages, Scrooge is deeply saddened by the possibility of Tiny Tim's death. He feels ashamed by his words that are thrust back at him by The Ghost of Christmas Present.
In this scene, Scrooge shows sincere grief for the Cratchit family and remorse for his past ways. Witnessing the absence of Tiny Tim at the next Christmas gathering helps him make the connection between his present acts and their imminent consequences. He implores the Ghost to intervene.
When he wakes up from his vision, Scrooge is a changed man. He is overjoyed to learn that it is really not too late and immediately goes about helping others, and in particular the Cratchit family. Both his deeds of charity and his inner joy are clear signs he is a different Scrooge from the "Bah! Humbug" one at the beginning of the story.
Indeed, he is devasted but, to use Dante-diction, he is precisely in the purgatory. You must not forget that A Christmas Carol is a traditional tale where the protagonist must move on from on stage to another; in Aristotelian terms: from BAD fortune to GOOD fortune. It is in a way a bildungsroman where the character evolves and discovers a truth to which he was blind. I say this because you must situate this passage of the novel with respect to these preceeding notions. Tiny Tim's death marks a crucial turning point. If you pay attention to the titles on the top of the pages, you will see that "Poor Tiny Tim" is followed by "TRUE sympathy" Hence, the 'devastation' is somehow relative. He is so in a way: it's a farewell to the old Scrooge. But the reaction to this death marks also the revival of Scrooge's humanity. I would say that the basic reaction is simply sorrow and sympathy. The two fuel the engine of the BIG and salient reaction: Change.