After you file

Did he get a W2 or 1099NEC for the part time job?  Being a full time student or under a certain age doesn't exempt you from income tax.  If he had withholding taken out it wasn't enough to cover the tax due on his income.  If he was paid as a 1099 independent contractor he owes about 15% self employment tax on it in addition to any regular income tax.  Why do you think it's wrong?  If he was under 24 he can be claimed as a dependent on someone else's return which disqualifies him from several credits.