As your son does not have infection because the blood work is negative. This is most likely due to muscle strain or other soft tissue sprain. This kind of injury can be caused by sudden force(whiplash) resulting from any accident or from straining the neck if neck is already stiff. treatment is medications, These could be oral medications, topical(apply over the skin in the form of cream or ointment to relieve pain) or in the form of injection, you could try analgesics such as pain killers like Ibuprofen etc. In addition, heat therapy could be beneficial and this is not very expensive. You can consult doctor of chiropractic in case of manipulation of cervical(Neck) spine if needed. Just to be on safe side as my first concern is safety of the patient, I advice you to go for X.Ray(AP and Lateral view) of Cervical spine. If stiff neck pain does not subsidize(decrease) within one week , then I advice you to seek immediate medical attention for further investigations to rule out any serious underlying cause. I think this would help you, Please keep record of every test and report.