Circle of Lightning adds a Damage Barrier to the enchanted unit.
Damage Barriers are active in Step 6 of the combat resolution.
"If the attacker made a successful melee attack on the defender, and the defender has a damage barrier, the defender may make a “free” attack with the damage barrier at this time."
So if you attacked your son's creature and it had a Circle of Lightning then after Step 4 Damage and Effects and Step 5. Additional Strikes your son would be able to attack back with a 2 Dice Electrical attack.
This can't be used as an offensive attack and is only active if the creature is hit.
Check out page 33 of the rules for more info.