Chihuahua male 11 years old

Hi all

Just looking for some advise. My chihuahua has an upset tummy at least every three or four days.

Before he was on cooked chicken and venison mince and although he sometimes had an upset stomach it was maybe once a month.
At the beginning of January I was concerned that he may not be getting a balanced diet eating just cooked meat and I put him on Royal Canine digestive care.
He then had an upset stomach every 3 to 4 days.My vet suggested I put him on hydrolyzed protein.
I started with Hills but he would not eat it after 3 days so we switched to Royal Canine.
This was on 25 Jan. He had a course of antibiotics at the end of January as well.
He has been on this food now for a month and he is still getting stomach upsets every 3 to 4 days.
If you give him any medication to relive the upset tummy he will vomit 3-4 times and last two will have lots of bile coming out.
The vet gave him antacid which we used last week.
He was sick again today after 4 good days and I did not give him anything. He seems to have recovered but only wanted some yogurt and not his food.
Now I am wondering whether I need to continue with the food or just go back to giving him cooked chicken and mince as he was not sick as often or do I continue with the food longer.

We also did a blood test this week and there is no abnormalities or concerns.

Any guidance will be appreciated