My son is 13 and has been having chest pain today he just broke out in a rash on his face and abdomen what could be wrong?

Chicken pox? hard to say without seeing the rash, but pain and rash can be from chicken pox, which starts out as red bumps on the chest that spread to the head and limbs and develop as small blisters filled w clear fluid. Chicken pox can have complications in adolescents so see a Dr - call first, they may want to bring u in through the back to prevent spread.
Allergy? In a 13 year old boy with chest pain and rash, I would consider an allergic reaction as well. As the prior physicians have answered, there are many causes of chest pain and rash, and he should be evaluated by a physician.
Mgt. Other causes include strep or staph, in addition to viral infection. I would suggest monitoring his temperature, breathing pattern and providing fluids.