TurboTax calculates the necessary estimated tax payments based on the assumption that tax withholding and, for state tax, state of residence will be unchanged and income will not decrease from one year to the next. If those assumptions are correct, the estimated tax payment vouchers produced by TurboTax show the minimum quarterly amounts the must be paid to qualify for the the safe harbor of paying 110% of last year's Maryland tax liability.
In your son's case, the penalty and interest imposed by Maryland indicates that his tax withholding was insufficient to qualify for the safe harbor of paying at least 110% of the previous year's tax liability, for the safe harbor of tax withholding totaling at least 90% of the current-year tax liability or for the safe harbor of having a balance due of $500 or less.
To avoid the need to make state estimated tax payments in the future, he likely needs to have his employer increase his state tax withholding.
