Feed in the cage there is no reason not to. With the aspen substrate I would watch for him ingesting any large pieces or large amounts but other then that you should be good to go. You said that he is in a smaller tank now, right? if not then something no larger then a 20G Long should do you good. Before spending a lot on enclosures I would look at all your options, I have switched to racks and love it. With just the one a tub would do you fine.
Also what are you using to regulate your temperature?
What are you using to measure temperatures and humidity?
I would try feeding live if he starts to lose weight. Get a good scale (food or postage) and keep track of his weight.
Do not handle him until you have a good couple of weeks of feeding.
Triple check all your husbandry, and just be patient he will come around.
Also if I am not mistaken Petsmart does not sell females so he is a he.
Care Sheet read 3X
http://www.ball-pythons.net/modules/...warticle&id=59
hope this helps a little. I am still kinda new to the hobby so if you have any questions I will help if I can, if not there are some great people on here that know a whole lot more than me.
Mike