Building up for a tragic let-down

Everybody who knew young David Hartman thought he was riding for a fall. Even his own family realized that the time to call a halt to Dave's impossible plan was that September night in 1968, the night before he first went off to college. The four Hartmans were lingering at the dinner table in their Havertown, (U.S.A.), home when David, who was blind, broached an old subject with a new intensity. "Father," he said, "tell me honestly, Do you think I can ever be a doctor?"

Fred Hartman, a bank officer and a very practical man, stalled before replying. It was one thing not to pamper Dave, quite another to let him go on building up for a tragic let-down. What meidcal college would accept a blind student? The time had come to set David straight. But, wondered the father, how cold he give a flat 'no' to a boy like Dave? How could he clamp a ceiling on his dreams? And so, Fred Hartman finally said, "A doctor, son?" Well, you'll never know unless you try, will you?"

Both he and David grinned. For this was the same response he had always made to David's "Can I do?" queries - ever since the boy, born defective lenses, had gone completely blind at the age of eight. "Dad", Dave hd asked at the age of ten, "Can I play baseball?"

Could you please explain to me the emboldened parts?

Though I know "build up" means "to produce", tragic means "terrible" and "let-down" means "fail in the endeavor".