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Since it has been two days, I would be concerned. If he seems well, you can wait until tomorrow to see the vet. You really have to go with your gut with things like this.
If he won't eat, seems lethargic, stumbles, or is unable to concentrate, it's pretty serious.
FYI- the most common cause of this is a parasite problem or giardia or coccidia. Your vet can give you appropriate meds to treat after confirming with a fecal exam.
Let me know if you still have questions.