The answer is I wish that he were already here now.
The subjunctive mood is a grammatical mood used to express a condition that is not a fact, but a wish, a suggestion, a command, a hypothetical idea, a doubt, a regret, a request or a proposal.
The verbs in this mood change slightly. For example, when speaking in the Present Tense, the subjunctive mood of the verb to be is "be", and when speaking in the Past Tense, the verb to be is "were". This is regardless of what the subject is.
This is why the chosen sentence is the correct answer. It expresses a wish and uses the subjunctive form of the verb "were".