He must decide who/whom to be. Which is correct?

Solution 1:

The Wikipedia link about the accusative case explains that

Modern English, which almost entirely lacks declension in its nouns, does not have an explicitly marked accusative case even in the pronouns. Such forms as whom, them, and her derive rather from the old Germanic dative forms, of which the -m and -r endings are characteristic.

Now, whether to use who or whom in your sentence entirely depends on which case should be used, accusative (whom) or nominative (who).

In English, it is grammatically correct to use nominative after the verb to be as in

It's he who stole my car. It's they who told me the truth. It's she who lied to me.

However, we know that "It's me" (using the accusative case after to be) is broadly used in English. But it is just a few exceptions.

In your sentence, it is appropriate to use the nominative case as it is the complement of to be. If you divide the sentence into two parts:

He must decide / He should be who => He must ask who he should be => He must decide who to be.

in the same way as:

He must decide / He should meet whom => He must decide whom he should meet => He must decide whom to meet.

He must ask / She is who => He must ask who she is. (This question cannot be shortened with wh-word + to-infinitive as the subjects are not same.)

We don't ask,

*Whom is he? or *Who is him?

*Whom am I? or *Who am me?

because whom and him/me are the accusative case and can't be a complement of the verb be in this case.

Note: "He must decide who he wants to be" is more idiomatic than "he must decide who to be".