What does the spirit mean will happen to tiny Tim?

This refers to the Ghost of Christmas Present when Scrooge sees Tiny Tim for the first time and noting his clear health issues asks "and what of the little lad?" The ghost states "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.''

No, no,'' said Scrooge. Oh, no, kind Spirit! say he will be spared.''

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.''

The ghost is clearly stating that if events continue without changes then Tim will certainly die, should these change then there is a chance he will live. The ghost makes its clear to Scrooge that Scrooges own Words will haunt him where Scrooge previously states when told many would rather die than go to the work house scrooges quotes is "best they do that and reduce the over population!".