Hi. I'm Dr. Lee and will do my best to help.
It is very possible that he has food allergy/hypersensitivity. I would suggest to get an over the counter diet called sensitive stomach, Science Diet makes such a diet. As well, I would suggest to give Beowulf some human over the counter Pepcid 10 mg tablets and give 1 tablet per 40 pounds of body weight, and give it every 12 hours. If after 1 week and there is no improvement, then you need to ask your vet for specific veterinary diets such as Prescription Diet i/d and z/d.
Please let me know if you have any questions or concerns. Thanks.
Yes. All of these can be related to food hypersensitivity/allergy. After vomiting, a dog will not feel good, and that is normal for them to "look sad". He can also possibly have inflammatory bowel disease, which will require surgical biopsy of the intestine to diagnose.