Well, I liked the concept.
I think others have covered the ground that the ending to this story is unearned and predictable, so I'll just say I agree and leave it at that. I also quibble with the writing. To highlight the son's alien body, his father is constantly making observations like this:
"He smiled, sadly I thought."
"he repeated with what I assume passed for a puzzled frown."
"He sighed deeply. At least I think he did. With those chimes I couldn’t be sure."
"I think he frowned. I couldn’t tell for certain, not with that face."
"He took her hand very gently and gave her what seemed like a wistful smile, though I couldn’t be sure."
The thing is, as far as I can tell, we're supposed to be taking these observations at face value, as accurate signals of the son's emotions. And so instead of highlighting the alieness, they detract from it. They feel tacked-on. "He frowned--oh, wait, he can't frown. Well, I'll just say it seemed like he did."
It would have been more effective, I think, if at the beginning there had been straight-on descriptions of what the son was doing--how his silver skin rippled, how his eyes shifted color, how his limbs twitched--with no commentary on what they meant. The addition of interpretation later on in the story would then have highlighted the father's growing empathy, would have read less as a magical ability to read emotion in an alien body and more as sympathetic projection. Which might have made the end less abrupt, too.