"Good and noble [man], [son] of a good and noble, just and charitable [father], may the Lord give you the blessing of heaven."
In Greek, the genitive alone, without υἱός, typically indicates paternity.
Καλὲ καὶ ἀγαθέ -- in classical Greek, these two adjectives together, frequently written Καλὲ κἀγαθέ with crasis, simply mean something like "gentleman," a man of upper-class or aristocratic family, and therefore naturally both handsome and good, qualities that were presumed to inherently characterize upper-class, wealthy men. The sense of good looks and moral probity has been watered down in this expression to a stock honorific. You might translate the vocatives here as "Noble sir", or "Distinguished sir" or maybe even just "Sir", and the genitives as "son of a noble man" (not "nobleman") or perhaps "son of a just and charitable gentleman."
the "of heaven" refers to the act of giving a blessing, but how do I know it is not referring to "κύριος"?
οὐρανοῦ is adjacent to εὐλογίαν, not κύριος. Although Greek word order is very flexible, here the word order makes it unlikely that οὐρανοῦ is dependent on κύριος.