Thanks all! We are headed to the vet this afternoon. At our very first visit about a month ago, I brought a stool sample but was told it was unlikely to be worms, since they don't have hookworm in California. I am new to the area, so I believed them at the time and they didn't test it.
He's been on the bland diet for nearly five days (mostly just overcooked white rice, plus probiotics once a day) and his stool is coming out bright orange and the consistency of hot coffee. Generally has an appetite, but is a little lethargic and still has some urgency to go to the bathroom. We live in an apartment building, but thankfully he is good about letting us know when he needs to go out.
While it may have started as anxiety, it is unlikely that he is still experiencing it. He's been resilient with changes before and I work from home so he is rarely alone (a trigger for him in the past). Will see what the vet says today. Very appreciative of all of the suggestions here!