Well, with respect to the verdict, about the most he could do would be to disapprove a finding of guilty and dismiss the Specification altogether. Never seen it done before in 39 years as a military and DoD civilian court reporter, but I guess it's possible. All that would do is make the accused less culpable.
With respect to the sentence, the Convening Authority does have to approve the sentence. However, the Convening Authority only has the authority/ability to DECREASE the sentence. If he disapproves it altogether, it becomes "no punishment".