The fact that he sees a problem here at all (how can one be a master and another a slave if they’re both responsive to reason and capable of fine action?) is your clue that he almost certainly doesn’t think of slaves as subhuman in the way modern slavers and racists sometimes did.
Charles Carroll’s “The Negro a Beast” is probably a useful contrast case
Popular 19th-century slaver ideology (and attendant racism) was by some distance more backward than Aristotle.
“Slave” was a social status rather than an inherent nature. If your ship got attacked by pirates you had a real good chance of being sold into slavery unless you were worth a random.
*ransom