"Let his come in" has 9 hits on Google.
"Let him come in" has 996,000.
So if your partner chooses 'his' instead of 'him,' he (or she) has a 1 in 110,667 chance of getting the job.

Since you want to "change the sentence to Simple Past Tense," I would put all of the verbs that are currently in the present tense into the past tense: "I found a pet store that sold ferrets."

'Octopus' is New Latin dating from the 18th century. It may be of Greek origin, but it's not Greek (if it were, it'd be spelled 'oktopous,' with 'oktopodes' as the plural. So 'octopi' is a perfectly fine plural, as is the anglicized 'octopuses.'