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If the stools your son is passing is smelly and watery it will be important to rule out infection;so first get his stools tested.Secondly stop lactulose
as it is not helping him in any way and you can start on a probiotic
till you get the report and you can use anything like syrup-Nutrolin-B or Rinifol
which is a combination of multivitamin
.If the stools revert to normal with the help of probiotic the fine or your son may need to take antibiotics if the stools test shows any sort of infection so take him to another doctor or Paediatrician.
Wish him a speedy recovery