I'm sorry to hear your dog is experiencing this nausea; we all know how uncomfortable that can be. Giving him the Pepcid AC, was a good move on your part. However, if the Pepcid doesn't seem to be helping, your dog will need to be evaluated by his vet, as soon as possible, to determine the cause of the nausea, and then resolve it with treatment.
Even though you don't see him getting into anything he shouldn't he could be nibbling a plant or something else, inside or outside, when he's not with you. Also, he may be experiencing an allergic reaction to some ingredients in his food, especially if it contains corn, gluten, soy, wheat, and/or artificial additives/preservatives.
For the moment, try feeding your dog small portions of bland food, like plain boiled chicken breast (nothing added) mixed with plain boiled white rice. You can also offer plain jarred baby food chicken (with no onions/onion powder). Mix 1 teaspoon of plain, unflavored yogurt into a small portion of these foods, to add good bacteria to his digestive system, and this might help with the nausea, also.
I hope he's feeling more comfortable, very soon!