Genetically speaking, a clone is most like an identical twin brother. So, Clark would basically be the kid's uncle, more or less.
Now, being the kid's "uncle" might grant him some status in a custody claim, but it would be up to the individual judge how to interpret the strength of that claim.
Now, the fact that you'd have an interested father (and one who would later be given top rating by an adoption screener) would be a help. On the other hand, assuming this is Season One we're talking about here, Clark would be a single father, which would count against him.
Not sure how things would play out if it was Superman asking. On the one hand, we'd clearly be talking about a child with rather unique needs, and we'd also be talking about Superman, international paragon of virtue and responsibility and all that good stuff. On the other hand, Superman is also single, has a very dangerous job, no known source of income, no known place of residence, no known family...
As for how he'd react... I think it would be pretty wierd for him, to say the least. He'd be happy to know that he is genetically compatible with earthlings, and I think on some level he'd enjoy having a child. It's probably fair to say, though, that the circumstances would be less than ideal. Not at like him planning to have a baby with his wife. Still, I can definitely see Clark quickly growing attached to the kid.
He'd also be able to count on help and support from his parents, of course, so that would make things somewhat easier.
Does that about answer your questions?
Paul, flashing back to an old sig of his:
"Boba, I am your father. Or, well, a clone of your father, which makes me more of an uncle. Then again, we're the same generation, so maybe more of a brother, or, I guess, half-brother.... Anyway, we're family, so please, don't shoot me!" - Last words of an uncounted number of stormtroopers
