Whats the best dry food to feed an overweight dog purina obesity mangement or royal canin obesity?

my dog who is eight years old and im feeding him purina om but he always hungry and isnt losing any weight on it hes been on it for afew years and his weight just goes down awee bit then goes back up to 28kg he also has raw blisters on his legs and im wondering if it could be an reaction to the food cause he didnt have it when he was on different food