My son had had hives for 4 months now. Just had the rast blood screen done and all came back ok. But he has hives everyday!?

Not surprising. There are many causes of hives, which can be difficult to diagnose. If a specific allergy is not obvious (e.g., hives get worse every time he eats something containing peanut), allergy is typically not the cause. Negative rast tests would be expected. His hives have lasted long enough that he should see an allergist, who is trained in the diagnosis and management of hives.
Hives. There are numerous other causes for hives (urticaria) other than allergies. There are such conditions as heat urticaria, cold urticaria, stress urticaria, pressure urticaria. Hives can also be a sign of systemic disease. He may also have eczema or atopic dermatitis or psoriasis.
Mgt/workup. I would suggest speaking to your PCP about the causes of chronic hives (urticaria). There are several treatment options. The RAST panel tests only certain allergens. Has your son been treated with steroids, anti histamines? Does he have a total IgE level? A virtual appointment is available online and the labs can be uploaded to his EMR.