How to express it in natural English: A dad regrets that his son doesn't live up to his expectations

Yea Ori 'tis, 'cause 'his' could mean either. Yet we'd say it that way anyhow. I suppose instead, 'That his son doesn't live up to his expectations, a dad regrets,' though terrible

I'd 'gest a better'n, but so many so much smarter'n me around....
S'ppose howevere, could say, '...A father...up to Dad's expec...'

Disappointment (adjective)
sad or displeased because someone or something has failed to fulfill one's hopes or expectations. "I'm disappointed in you, Mary"

synonyms: upset, saddened, let down, cast down, disheartened, downhearted, downcast, depressed, dispirited, discouraged, despondent, dismayed, crestfallen, distressed, chagrined; More

Kids always disappoint their parents. I wanted mine to be the new Scooby Doo gang. Have they done it? Have they ****. I can sort of understand about the talking dog, but they haven't even got a ******* van!

Scooby Gang is a good clue. But it is relatively new. Graceful wording without f* will be more appreciated. It is best that an English idiom can express it.

Expectations lead to disappointment/suffering.