I have been told that you CAN take a probiotic if you wish while taking a course of antibiotics, you just have to time it well. For example, if it is a one-a-day capsule then the probiotic would go down 12 hours after the antibiotic did.
Tell the BF that I hope he feels well soon. I suffered from recurring tonsil infections and felt like a pile of crap that then got run over by a train, until I eventually had a tonsillectomy at 24 because the tonsils had by then become rotten right to the core. Not fun stuff.
I would just do the usual health-building stuff: sun and fresh air, sleeeeeep, liver, fruits and veggies, bone broth, fermented foods, water, fresh herbs on the food, sea vegetables and fatty fish. And definitely tons of fermented foods after the antibiotics have ended.