From my own experience regarding RC food it is not good quality food , i would stick with what you were feeding -what consistency are his poops ?
Your pup is still young and will make mistakes and my first bulldog was the worst in potty training -
Also it could be the fact that he is locked up while you are at work for a bit to long (even tho you got someone taking him out) and his getting a bit stressed
Maybe have someone stay and play with him for a while ?
Pups this young should be feed 3-4 meals a day rather than 2-they are growing and have faster metabolisms (Like children) and need to eat more often
I hope more members who were potty training while working will be able to advise you how did they manage