I am a small employer. I need to provide an employee who is resigning a w2. I know how how to complete the W2, but I am confused about box 12. I plan on placing in box 12a the code DD amount. I have no further information needed in box 12.

Is it acceptable to only have code DD in box 12. And the amount, with no other information in box 12b - 12d, if none is required?

yes; Note that this code is for reporting purposes onlyDD reports the cost of non-taxable health insurance provided through your employer. So it is employer-sponsored medical plan. It is not used for income tax returns.