I think the reason why his son died so early, and why male offspring was so problematic in general was obviously the genetic inadequacy of Henry VIII...
Actually, I've read Katherine of Aragon was fond of religious fasting even while pregnant, and this contributed to her many miscarriages.
It might have also contributed to the weakness of her son.
Of course, I've also heard Henry had syphilis, which can be hereditary. It would explain why Edward VI was so sickly.