He may have an upset tummy, but I would definitely watch him for signs of improvement- soon. If not, a vet trip may be in order. Trust your gut instincts if you think he is not just having a tummy/flu bug issue.
Have you been anywhere lately where he could have gotten a dog illness? Has he drank out of a puddle, eating any plants/grass?
If you have some pedialyte you can add that to chicken or beef broth to get him some fluids. You can also try burping him by patting his back with firm 'slaps' and see if he has air trapped in his tummy.