diarrhea from IV antibiotics

My husband recently had a hip revision done after just 3 years. They found c.acne bacteria on the old joint stem (after the revision was completed) and so he is now on IV rocephen daily for 6 weeks to be followed by 6 months of oral antibiotics. He is now on day 12 of the daily IV antibiotics and he has been experiencing worsening diarrhea (every 3 or 4 hours) It does not seem like c diff (no fever, chills, cramping or pain or odor) the PA told him to try a BRAT diet but it isn’t helping, nor is is sustainable for nutrition. Any ideas of how to help? He is miserable and I am concerned that his health will suffer if this continues.