Toss a live mouse hopper in there and watch this feeding issue become a non-issue in about 3 minutes.
Getting them started on live doesn't mean you can't switch them over to F/T later on. But, you need to get him started first.
P.S. Any pet shop that would sell a ball python that young and unstarted, is uninformed at best, and down right shady at worst. Either way, I would never darken their door again.