I understand your feeling my grandma is always worried about me and keep telling me to be safe even though I am 31 yrs old now, so I do understand your concern as the kid is just 6 yrs old.
Let me explain you the stuff going on and what we mean by the enzymes and all heart things in simple form so I can.
(A) EKG- Its an graph is plotted on the paper with sensitive instrument which can detect the electric signals passing in heart, ECK normal that means the heart is function normally.
(B) OK the enzymes are used as markers to trace the level of damage as its not visible by naked eye, but the level can be seen in blood, its like if you mix salt in a water its not possible to say that water is salty but only can be identified by taste or some chemical test in the lab.
1) ENZYMES are the chemicals which are released by the dying cell when its ruptured, same way if we crush the can, the soda comes out and spill in the floor, in the same way enzymes comes out the cell and get mixed in blood stream and those enzyme quantity is measured and can be used to estimated amount of the damage/ number of cells effected. "CREATINE
PHOSHO KINASE (CPK)" is one of such enzyme that is released by muscle cells as well as the heart cells - so the "CPK-MB" is specific enzyme for heart where as the other CPK - is released by the muscle cells,
2) Now about the TROPONIN- These are the component structure of heart cell (small protein) only found in heart cell , which is released in blood when the cell is ruptured so it can be an indirect measure for damage to heart cell, so measuring the TROPONIN -I , TROPONIN-C, TROPONIN-T (various kind of component) are measured to check access the damage in indirect form.
Now the amount of CPK enzyme will remain blood is approximately 8-12 hours after release and later on it will be destroyed by liver
, while the TROPONIN STAYS FOR 6 Hours after the release, so the amount of "CPK- will remain elevated for some time even if troponin comes back to normal.
The best judge for heart damage is TROPONIN - if its normal that means the no more cell are dying in heart
Were as the High level of CPK-ENZYME means it can be due to muscle cell damage
In short the boy don't have any more issues with heart.
Hope it helps.