He may have eating some non food item that is causing a blockage or a partial blockage. Socks, rocks, sticks, rawhides, toys all can cause a problem like this.
You may want to have a vet check him and maybe do an x-ray to see if there is something physical causing this intestinal upset.
It might also be a symptom of intestinal parasites such as coccidia or giardia particularly if he has had a drink from any open water like puddles or ponds or streams.
A stool sample check would give you an answer on that.
A blockage or partial blockage is serious though so if you recall him chewing something or getting into trash you might want to have vet check him today.
Take a look at his gums and tongue to see if they look pale or bluish as signs like that would be a strong indicator seeing the vet today would be best as would serious abdominal pain or lethargy.
Hope this helps you!
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