Actually the 1120-S and the Sch C are very much the same ... so if you understand one you know both.
Now ... the cost of the purchased assets get depreciated or expensed whichever you choose to do. Other things paid with the loan are expensed. And the repayment of the loans themselves are not deductible (since the items purchased with the card are already reported) that would be double dipping ... only the interest paid is an expense. Paying off the loan/card to reduce interest expenses is not a bad idea if you can do it.
I highly recommend your son to use a local professional to get educated on filing the 1120-S, get the books set up correctly and learn about the payroll requirements for owner/operators ... failure to report wages for the owner correctly is the #1 audit factor for S corps.
