Regurges can happen at any age, so it's not unusual for him to have his first aged 5.
In an established feeder it can indicate a) He was disturbed or upset in some way whilst in the early stages of digestion, b) The food item was too big, c) He's ill.
The first two are the most common causes of a regurge by a long way. If there are no other signs of ill-health, then look at how he was treated after eating and the size of food.
Agree with above: 10 days before the next feeding attempt, then offer a smaller food item than he'd usually eat (in your shoes I'd try a medium or even small mouse).
Large mice are the normal food for adult Corns - they just don't need anything the size of a rat. Once he's eating properly again, go to one large mouse every two weeks and see how he does.