The idea that the Howl's moving castle movie is just the embellished story told by Howl to Morgan while Sophie threatens to cut off his suits again because: "That's definitely not how that went, Howl ! " is hilarious.
The fact that Howl lightheartedly called his new born ugly and proceeded to say "chip off the old block" and got yelled at by Sophie means that Morgan is Sophie's spitting image
It is the tragedy, the Aristotelian and comprehensive tragedy, of sin coming home to roost. That is why we have to take note of the parentage of Arthur's son, Mordred, and to remember, when the time comes, that the king had slept with his own sister. He did not know he was doing so, and perhaps it may have been due to her, but it seems, in tragedy, that innocence is not enough.
One really cute thing about Howl that I really really liked is that he canonically is able to hold a newborn.
This may not seem so significant until you remember that Sophie got freaked out when trying to hold Morgan for the first time, like she was geniunely scared of holding him and didn't really know how. She's the mother. Typically you'd expect the mother to know what she's doing and the father to be absolutely clueless, especially when the father is someone like Howl.
However, instead we get the reverse; Sophie is the uncertain panicked one and Howl is the one who is clearly more comfortable and I believe it was said he looked more used to holding babies too.