The classic case of " Drug Induced Diarrhea". Since you have stopped the antibiotics, the guy will try to recover on its own, which may take time.
Quick fix: start a probiotic ( Enterogermina , a vial to be taken in the morning and evening for 3 days, pls refrigerate this medication). Or Sporolac ( available as sachet or tab).
To heal the gut: simple fresh home food, hydrate very well, replenish
electrolytes ( ORS, coconut water, Kanji, chaas, nimbu pani).Repeat on loop for 48 hours.
you are loosing
electrolytes which makes one feel drained- replenish those and bounce back.