In Standard W(ritten, hite) English (hereinafter abbreviated SWE) there are two main issues here.

A) Be can be the infinitive (yes, Virginia, there are forms of the infinitive in which the to is absent as it is in this sentence (the third word in this sentence to be specific).

2) It can also be an example of the subjunctive in what is known in Latin grammar (valid here for English also) as the hortatory subjunctive, often used with the periphrastic let. This is the normal form of the polite command. Other examples of the subjunctive would be if he be here he can defend himself. This use of the subjunctive normally requires that the clause in which the subjunctive is used is known to be contrary to fact. Thus, if it is known that he is here or if it is not known whether he is or not, the proper usage would be If he is here he can defend himself