The body of your son has defended itself from infection by localising the same to mesenteric lymph nodes. The lymph nodes will eventually destroy the infective microbe.
Usually when a child gets nasal or throat infection
the post nasal drip
or the child un-intentionally swallowing the nasal discharge without spitting it out transmits microbe to throat and onwardly to intestines would lead to loose stools and the severe pain
on abdomen due to mesenteric adenitis
is also called "TONSIL TUMMY" by ENT surgeons. Tonsil Tummy mimics acute abdomen
.The omentum/ and mesentery are also known as Policemen of abdomen and it defends the abdomen by invading bacteria and viruses.
In any case a course of Anti biotics and Anti inflammatory as prescribed by your treating doctor will cure the Condition .No need to worry.
Get well soon.
Dr Lt Cdr ASN Bhushan.