It could be that the test result was really low. Was it a blood test or a skin test? Did they provide you with the numbers, if it was a blood test? If it was really low, and he never had any reactions before, they may believe it could be a false negative, like previous posters has said.
I'd just be careful when giving it to him. Like during the morning, to allow you time to watch him to see if anything happens, be prepared (dressed/showered/etc) in case you have to take him to the doctor.