CP, and the Yorks Visitation, are unclear about this chap's parentage.  He may have been the son of John, son of John and Jane Reresby.  Or the intervening John may not have existed and he was the son of the first John.  Or his name is John not William.  I cannot make sense of the words in CP, nor can I correlate them with the Yorks Visitation of 1563-4, p. 123.

He had brothers Wm. the eldest who inherited Sprotborough and Edmund, the ancestor of the earl of Southampton.  But the line to this earl is, as yet, indistinct.

CP recommends "Baildon and the Baildons" [vol I] pp. 343 et seq for a good pedigree on the FitzWilliamses.