Your son could be seronegative when testing to detect infectious conditions, causing the mesenteric lymphadenitis.
Suspect are infectious agents like Pathogenic Mycoplasma and their Co-infections (borrelia, bartonella, babesia, ehrlichia...), which makes them notoriously difficult to detect by blood tests.
They hide deep in the tissues, evading both detection
and the effects of antibiotics.
Also having the ability to envelope themselves in cellular membrane from the host's own cells, the even evade the immune system!
For this you would have to contact an LLMD and the testing to be done by IGeneX Labs.
Oh, could you detail ALL his symptoms?
And do you have a cat in your house?
But first you need to ensure that the most common cause, Yersinia, has been 100% ruled out!
For relief, your son can try some demulcent herbs, which soothe and protect inflamed internal tissues, like aloe vera, slippery elm, licorice, etc.
Until you get all this sorted, having your son on an SCD
protocol, may prove beneficial.
It is definitely worth looking into it.
My daughter following a case of salmonella, suffered similar symptoms to your son and after doing the medical merry-go-round for a couple years without a firm diagnosis, decided to do the SCD program(a modified version) and that eventually gave her back her...life!
Not 100%, but at least she's leading a functional life, finished College & University and is now a Holistic
Health Practitioner and Yoga instructor.
Please let me know what you think and also post your answers, when you have a chance.
Best wishes,
Niko