Live fish such as guppies, goldfish, and such are not good in the long run for him. They will lead to fatty liver disease and cause death. They are ok on an occasional basis as a treat and to start him out but you will need to convert him over to frozen meaty foods. There are many different methods to do this, if he isn't converted already.
One is to soak his live food in selcon, zoecon, or garlic before feeding to him. Then slowly add in silversides soaked in the same. ONce you have him eating non-live food, you can try things like mysis shrimp, krill, or any other meaty food. You may have to soak them as well at first to get him to take notice.
Another is once you have him settled in your tank and eating the live food, you can skip feeding him for a couple of days. Then get some silversides (I have to rip mine in half since they are too long for him to swallow in one gulp) and dangle them at the surface of the water. When he looks interested and about to snatch it up, let go and he will grab it. Let him digest for awhile and then give him another. This is the method that worked for us.
Another is to get him used to a feeding stick. Place the live food on the stick and have him come to the stick to grab it. Once he is used to this, switch over to frozen foods on the stick.
One thing that will help is to find out what the pet store was feeding him. Hopefully they had him eating frozen and you won't have to worry. If they were feeding live, you will have to condition him to frozen once he is settled into your tank. HTH Rhonda