Formula vs. Breastmilk... Help!

My 4 week old boy primarily gets breastmilk. I had a dip in production a week ago and gave him Similac formula, we had given it to him before following his birth until my milk came in. We had no problems initially but after this last time he had bloody/mucus diarrhea. We took him to our pediatrician and he said most likely a cow's milk protein intolerance. Told us it would take approximately 2-3 weeks for the protein to move through my system after giving up anything with cow's milk in my diet.

I've been watching my diet but my pediatrician also gave us Similac Alimentum. I wanted to try it yesterday... Initially he seemed fine, we gave it to him twice but all last night he was grunting and fussing like his tummy hurt. I gave it to him again this morning and he can't nap because he is writhing around like his belly hurts still.

I'm honestly at a loss here, I don't know whether to keep trying the formula and give it time or switch back to my breastmilk which probably still has cow's milk protein in it.

Side notes, I'm going back to work at 12 weeks and I won't be able to pump due to my job. I also want to get the right formula before my SO goes back to work in a few weeks.

We do have a one month check up on Tuesday, but I'd love to hear other Mama's experiences and what you did?! 😩