If you look at that first report of increased family income, also in the referenced Pew report it says 59% of sons make as much or more then their fathers, which means 41% are doing worse. So 4 of every 10 sons does worse than his father in this time of economic growth.
Two more factors come into play. US working class stagnation started in the early 1970s and got going in the 1980s. The study starts in 1968, which means some of the growth pointed to for the children is actually what is kept from late 1960s and early 1970s growth, not now. If the study had started five years later, the results would look worse.
Also the politically influenced Boskin commission revised historical inflation estimates in the mid 1990s, also making things look rosier. If you believe, as I do, that inflation estimates were correct in 1996 and that Boskin was wrong, then things look bleaker in that light as well.
Although 4 in 10 sons doing worse than their fathers (from the Pew report he cites) is bleak enough.