Small pinky mice should be fine. Thaw them out in a plastic baggie with some salmon or trout in it to marinate the scent in.
Silversides (smelt) can be bought by the bag at many places that specialize in selling tropical fish. Since they are whole, you wouldn't need to supplement them with any powders.
I am not an expert on earthworms - I buy mine from a bait shop that has assured me for years that they are not red wigglers. I tend to feed them lettuce and other veggies for about a week before I offer them to my snakes to clean out any gunk they may have in their systems.
Try the silversides. Thaw a couple out in tepid water until they are room temperature - you want to make sure that they are still not frozen. Offer them with a pair of feeding tongs. If they are too big, cut them into pieces.
If you can get him on silversides, feed him three weeks of the month on them and then try the pinky mice. The earthworms should be an occasional treat.
I think a corn snake is a great choice as a pet. As a rule they are generally docile, forgiving of small husbandry mistakes and disinclined to bite. As with any species, there are exceptions to the rule.............