Here are a couple of options:
Playing on the word "father":
* Downcast father, / Shattered laughter. (This emphasizes the contrast between the father's mood and what one might expect)
Focusing on the emotion:
* Downcast father, / Deeply after. (This uses "after" as a synonym for "sad" and makes it rhyme)
Using imagery:
* Downcast father, / Fading laughter. (This uses "fading" as a metaphor for the diminishing happiness)
Ultimately, the best rhyming pair depends on the specific context and what you want to emphasize.