I've found him in 1851 listed in Ross. Note that Susan is listed as stepmother, not mother
William Hook Head 30 Ag. Lab Ross
Emma " Wife 27 Abergavenny
Eliza " Daur 4 Ross
Anne " Daur 3 "
William " Son 8 m "
Susan " Stepmother W 50 Kings Capel
HO107 Piece 1976 Folio 150 page 13
In 1841 I've found another William Hook living in Ross aged 60. with him are James Hook 12 and Thomas Hook 9. I wonder if this is his father? I can't find the 1821 William though.
There is a William Hook and Susan Bailey on the same page in 4th quarter 1842 Ross Vol 26 page 316 marriages. This would be my educated guess for your William's father, for what it's worth
Anne's birth certificate is 1st quarter 1848 Ross Volume 26 page 217. I suppose that would give Emma's maiden name, then you could (hopefully) track their marriage certificate, and then William's father. That's getting expensive though.
Isobel