Yeah like kaorte said, pinkie mice wouldnt have been even close to big enough for any ball python, but we all make our mistakes.
Its great that he ate for you and I know how hard it is to let them settle in and not hold them for there adjustment period, but its for there own good. PAITIENCE is key in a this hobby.
I believe you will be able to switch him over to f/t or p/k once he gets a little older and a few meals in him. Congrats on the new addition eating though.