whats best kind of dog food

weve had Bandit for 3 yrs now he’ll be 4 in Nov, so i fed him puppy chow when he was a baby then put him on dog chow seemed fine then we moved in with my parents & 2 other dogs so he started eating their food dads brand he ate it so i didnt thnk alot of it.. he farts all the time so recently within the last 6 months he seems like his skin is super dry and flakey he still farts all the time well his skin looks irritated its red between his paws and his tummy. the boyfriends sister (a vet tech) says hes probably allergic to his food so we swich him to what we thought was good Purina Pro Plan sensitive stomach & skin.. that was 5 months ago and his skin is more dry than ever hes shedding worse than ever and now even his skin on his tummy is red.

what do we do to help him i feel so terrible for the poor fella. can someone ttell us what we should be feeding him please?