My son is 8 years old and has welts/hives that show up like scratches. he also have joint stiffness mainly in his hands.

If joint stiffness : has lasted a week or more, his pediatrician will want to see him. You'll be asked if he has redness, swelling or pain in his joints, trouble opening & closing his hands or flexing & extending his fingers, fever, & other changes in his health. Spontaneous welts may be related if he has some type of childhood arthritis or his body is reacting to a virus or medication in that way. Have him checked.