Sorry your little guy isn';t feeling well.
If I understand what you are saying is he was fine on Monday when you went back? (no more diarhea?) then he had shots and changed food and more antibiotcs? If so, it is possible the shots or the antibiotics or the food could be the culprit.
I don't think I would be comfortable with a vet who gave additional vaccinations after my pup already had them (actually , I wouldn't allow it or tolerate it if they did it without my ok! ) Did they ask you if that was ok with you first?
It is very possible to get diarhea off the hills ID food. I had a Bichon who got diarhea from it
, not saying your little guy did, I would be looking more at the excessive vaccinations as a reason, especially since you say his diahrea returned after the shots.
Do you have another vet to talk to for another opinion?