What made Bob Cratchit emotional about Tiny Tim?

When the Ghost of Christmas Present shows Scrooge the Cratchit family, Bob's voice becomes "tremulous" when he tells his family about what Tiny Tim said on the way home from church. He says that

"he hoped the people saw him in the church, because he was a cripple, and it might be pleasant to them to remember upon Christmas Day, who made lame beggars walk, and blind men see."

This is quite a remarkable statement for a young child to make. Tiny Tim does not feel sorry for himself, and he does not try to hide the disability caused by disease (possibly rickets, a severe vitamin D deficiency, which makes the bones soft and brittle and would be treated with the kinds of braces Tim wears). In fact, he wants people to see him because it would remind them of Jesus and the miracles Jesus performed, and this would be appropriate on the day of Jesus's birth. It's a thoughtful, generous, compassionate, kind thought from a child who has so many reasons to feel sad instead. This makes Bob quite emotional, as he recognizes how very special his son's heart is.

Bob Cratchit is a devoted family man and is especially attached to his youngest child, Tim. The boy suffers from an unspecified disease (possibly rickets) which has left him unable to walk without help. Dickens tells us that Tim generally walked using a crutch "and had his limbs supported by an iron frame." Bob carries his son much of the time.
Bob would like to get medical treatment for his young son but cannot afford it.
In Scrooge's visions, he sees two versions of the present and future. Though poor, the Cratchits enjoy their Christmas together, which surprises Scrooge. In one version of the future, however, the lack of medical attention has taken its toll, and Scrooge envisions the family's mourning following Tim's death. In that vision, the Ghost shows Bob crying and lamenting for his little son. It turns out that was just a vision, and Tim is still alive.