Yearling Fast - December

So i have a yearling, my first ball. He was eating 2 mice per week, then switched to rats. He ate rats every 2 weeks for a total of 2 rats. Then all the sudden, he stopped eating and taking interest in December.

Should I start to worry now? Maybe try going back to mice?

Has he lost a lot of weight? Is his spine visable beneath the skin? I would make sure husbandry is right on with temps, humidity etc. If he still looks healthy and plump, I wouldn't worry about it too much, a friend of mine had a full grown ball go a full 11 months to a year without food, then randomly decided it was time to start eating again. Now if he has lost a substantial amount of weight (not just a few grams) and acts lethargic, it may be time to hit up the vet. Regardless, I would double check temp, humidity, and even hide size, a lot won't eat if they don't feel secure. This is just my personal opinions and experience.