cat food allergy

my kitten 5 months old had a bout of vomiting and diarrhea[ one day of vomiting]
We went to our vet and he was on flagyl and pepcid for 5 days
he has been on hills i/d for almost a month
when I tried to transition him he got the loose stools back
solid gold wet and dry

I would like to get him on to natures balance duck and pea

any suggestions as to transitioning him now

Transition very slowly. Cats without sensitive stomachs should transition over 5-7 days when changing foods. Since yours has sensitivities I would slow that down to 2-3 weeks.