If he has been eating regularly since the end of April, then I'm sure he can handle larger prey. Eventually you will want to get him switched to rats, but considering he is probably quite underweight (as most pet store snakes are), I'd continue feeding mice as long as he accepts them. You would not want to risk him going on a feeding strike by offering a new prey item. Once he's put a bit of weight on him and begins looking healthier, you can try switching over to rats.. this way when he gets bigger, you will not have to feed multiple mice, rather an appropriately sized rat.
He's probably being a bit defensive for the couple days following a meal because that is the approximate time they require to digest their food. Mine do not move at all for the day or two following feeding night, then become active again once they have digested. I'm sure you already know this, but be sure you leave him alone for 48hrs after feeding to ensure he does not regurgitate his meal.
Hope I've been helpful.![]()