Per Registering to his classes, he SHOULD have gotten a confirmation of it and a payment receipt.
DOES he have these?
On his Registration form, he WOULD have seen, what classes he IS signed up for. And his payment receipt, will have shown that that class was paid for, or not.
How the heck, did he even take the class if he was not signed up for it?
A Professor, has a class list.
His name should have been on it.
If he was in that class, and the Professor's class list did NOT have his name on it, the Professor would have told him.
Thus, that would mean checking with the Registrar.
It is either a mistake of the Registrar's office or your son's mistake.
So, the school, DOES have records of who registered and paid for a class.
DID your son, check with them?
It could be 'their' mistake.
Bottom line is: IF your son has his Registration papers AND payment/tuition receipt... then, that will prove, he did sign up and pay for that class. Then, in that case, it would be the School's mistake, or the fault of their computer registration system.