According to Hindu scripture or shastras, what portion of his earnings should a son contribute, to the parents or family?
Does this amount vary if the son is married vs. unmarried?
I'm unable to find the exact source on Google, but I remember reading it long back, so moderators may delete this comment, or if anyone remembers the source, you can edit.
1/6th of a man's earnings is taken by King for taxes. Of the remaining 5/6, a man should allocate 1/6 for savings for times of distress, 1/6 for charity for poor and handicapped, 1/6 for daily needs for himself and family (includes wife, children and parents), 1/6 for bandhus (brothers, sisters, relatives in need, friends asking for loans etc.), 1/6 for personal pleasure, and i forgot the last sixth.
Of course, this mainly applies to olden days when mostly vaishyas (merchants) used to earn and they would support brahmins (scholars) and kshatriyas (warriors) by alms and taxes respectively. There was also not much distinction between caring for wife/children vs. caring for parents because they were all part of the 'family'.
Not sure if this is relevant to your question, but once a man is married, scriptures state his wealth is shared equally by his wife (she has to 'admit' every expenditure, including donations, by sprinkling water, as you can see in almost every Vedic ritual priests conduct).