may be primary or secondary.
Primary lactose intolerance is due to congenital deficiency of lactase enzyme, which si a very rare entity and usually manifests since birth. Secondary lactose intolerance occurs commonly after diarrhea
due to transient loss of cells producing lactase enzymes.Presence of reducing substance in stool indicates lactose intolerance.
Only mucus doesn't indicates infection as it may be present normally. As he is also symptomatic your doctor has prescribed him antibiotic.
You can also give him syp zinc for 10 days and enterogermina
for 3 days.
If oral acceptance is poor or he is getting lethargic consult your doc immediately.