What Causes Gastritis And Abdominal Pain In An Infant?

my four months old boy was diagnosed with secondary lactose intolerance at his age of 1.5 months. Now he feeds low-lactose formula and always suffers from gas and abdominal pain. His doctor advised Enterogermina twice daily for 14 days at the time of diagnosis of the intolerance i.e. at his age of 1.5 months. Aagain, my baby has gone through a course of Bifilac for 14 days at his age of 3 months. But my baby is still suffering from the problems I have mentioned. What should I do? Should I follow another course of Entergermina? Or, Bifilac? Or should I start solids?