Hello,
It is difficult to make diagnosis and give specific recommendation on internet but generally speaking, very high levels of creatine kinase (CK) like 13,000 are usually caused by rhabdomyolysis that is the destruction of muscle cells. This destruction release CK (that is an enzyme) to the bloodstream and you can detect the concentration in a blood sample. This problem can be generated with or without exercise depending on the cause. If there was no trigger for this problem like medications or systemic infections, a possible cause in a pediatric patient is the presence of a metabolic myopathy. There are several types of myopathies but in general they interfere with the muscle production of energy, and during physical activity (when the demand of energy is high) can produce destruction of the muscle cells and high CK levels. My suggestion would be to try to avoid your son of doing intense physical activity (if that is possible in a 7 yo kid) and keep him very well hydrated and with good potassium and magnesium intake (e.g. bananas, coconut juice, artichokes, nuts). The workup and different tests will probably take some time so be patient and talk with your doctor if new symptoms develop or if you have new concerns.
Good luck.
"struggling to walk up stairs"
In what way? short of breath? muscle pains? chest pains?
I believe Aldolase has three types, A B and C. Do you know which one in particular was high?
Is your Son normally very active, does he play a lot of physical sports?