Exactly. It's not about smell; it's not about "otherness". It's just a rival male. The challenge and the appropriate response would be exactly the same if he'd raised it from a cub in his own pride.
What happens to the innate racism?All strangers are perceived as threats, initially. That assessment may be revised with further interaction.
Few cultures is some cultures. Even if that "few" were true, two contiguous cultures - the ones most frequently at war or raiding each other - are more likely to be the same race as not.[What matters is that the boy is his property, to do with as he will. ]
Few cultures allowed enslavement of their own members by other members, except as a punishment for some offense.
Or the child was sold for debt, or born in the wrong caste, or being captured in war. Or his mother is a slave and the owner routinely rapes her. Or she was a gift/tribute concubine performing her duty. All quite common situations.
They don't have to differ. They just have to be labelled. By law - i.e. government.T[here is nothing inherent or biological in these relations: they're the product of arbitrary social designation.]
What is inherent is the propensity to develop different social cognitions with respect to those who differ from one in various obvious ways.
Droit du seigneur.