"Wees een man!"
Translation:Be a man!
Yes, it's the imperative of the verb zijn, so there is no other way of saying this. For most verbs, the imperative is the same as the conjugation for ik, but zijn is an exception.
The only other possibility I can think of is using the subjunctive. It's still common in German but it has become so archaic in Dutch that I wouldn't recommend it. Unless you want to sound archaic on purpose. Like you could use ''thou'' and so on in English.