How old would Ahaz have been when his son, Hezekiah was born? A cursory look at 2 Kings 16:1 and 18:1 suggest he may have only been eleven. (Ahaz was 20 when he became king and he reigned 16 years. So he died aged 36. Hezekiah was 25 when he became king. 36 - 25 = 11)
Would a more likely possibility have been that they were brothers - both of them sons of Jotham?