Hi,
Thank you for your consult at DoctorSpring.com.

Does he has fever/ diarrhea/ constipation/ bleeding per rectum/ vomiting and nausea?

With the available described details, there appears to be high chances of infectious etiology.

Splenomegaly with enlarged lymph nodes could be associated with viral and bacterial infections. Epstein Barr virus, CMV, Yersinia enterocolitis, Campylobacter jejuni infection could be associated with defined pathology.

You should consult with Pediatrician and hematologist simultaneously and should go for thorough check up.

He should go for complete blood count, erythrocyte sedimentation rate, peripheral blood film, Quantiferon TB gold analysis, serology analysis of EB virus, and CMV virus.

As the disease remains undiagnosed after so many investigations, exploratory laparotomy with excision biopsy of mesenteric lymph nodes appears to be necessary.

Culture sensitivity of excised tissue also appears to be mandatory.

Treatment depends on exact diagnosis.
Meanwhile, he should take broad spectrum antibiotics i.e. quinolones, azithromycin, and ESBL sensitive antibiotics under guidance of treating doctor.

Take care,
Let me know your other associated query.
Wishing a speedy recovery.
Dr. Mayank Bhargava