My son had an allergy test today and it came back negative to almost everything. Why is he reacting?

???? symptoms??? Many people assume that nasal congestion is due to allergies. You fail to realize that the nasal membranes react for many reasons. Sensitivity to airborne fumes/dust/cleaning odors/fragrances/etc are common. There can be structural issues in the nose. If there are skin issues that also can come from many sources. We can help you more when you provide more detail about the problem.
Lots of reasons. If the allergist just said, "he's not allergic; I can't help you, " you need to see a better allergist. Good allergists take a more comprehensive approach to upper & lower respiratory symptoms, rather than just rely on skin tests.
? What kind of problem you are dealing with? Not all nasal and respiratory problems are related to allergy.
Allergy test. If done as a blood test can be 80% accurate skin test is more accurate. Also he should have immune evaluation if IGG or IGA deficient. He will act allergic but will not show test positive for allergies. He may be having problem with with vaccine recognition deficiency and have recurrent sinusitis of coarse anatomical factors like enlarged adenoids and sinus ostium blockage must be considered.
MGT. I would suggest a virtual appointment and uploading the test results to your records. He may have allergies to an allergen that was not part of the test panel, or another diagnosis.
"Reacting"???? "Reacting" is not a diagnosis. Why the allergy tests? What did the Drs say? So the question has no meaning and no answer. Submit the question again with a description of the exact SYMPTOMS.