If he co-signed then he 100%, absolutely took on the financial obligation. If you don't pay, he's legally, 100% on the hook for it.
He could not, and does not to this day, have the money to pay down those loans (which I am currently paying down). However, he did have one thing he could offer: a reasonable credit history.
Essentially he lent me credibility. (And then told me nobody would ever find the body if I didn't pay down my debt.)
Think of your stepdad's stance as tough love and motivation.