There is no medical test for intolerances. If SPT indicated a positive, and you've seen a reaction when he's had dairy, then you should avoid ALL sources of dairy. Coughing might be asthma,and is often seen in kids with food allergies, but is separate from the food allergy. Dust mite definitely might be the culprit. Have you mentioned the coughing to your allergist? And the coughing he had after the ice cream, was it at all like him having a hard time breathing? Any other reactions, like hives, swelling around mouth, lethargy, vomiting, diarrhea?
Gtg, sorry post later!