Loose motion in 4. 5 months old baby.

My baby is born premature,  now 4.5 months old. He is on exclusive breastfeed and is having loose motion from last 10 days. For first 3 days it was watery. We consulted doc , he gave bifilac gg twice a day and Z n D once daily 0.5 ml. Now the stools are not watery but still very loose and seedy and have bubbles. Stool frequency in 24 hours is between 3 to 6 times Stool test report shows ph 6 and reducing substance positive Now doc has suggested to stop breastfeeding completely and give formula milk which is lactose free. He has no other symptoms, is active and gaining weight steadily. His current wait is 6.3 kg. Please suggest if breastfeeding should be stopped and what to do regarding loose motion