Learn Italian: Verrà anche lui? Può darsi.

Yes, as a standalone reply they mean the same thing and are interchangeable. The key difference is how they behave if you expand the sentence: forse takes the indicative mood, such as forse verrà, while può darsi che triggers the subjunctive mood, as in può darsi che venga.

Why is the subject lui placed at the very end of the question?

Placing the subject pronoun after the verb puts the spoken emphasis on that specific person. It highlights whether he specifically will be joining, which sounds much more natural in conversational Italian than starting the sentence with anche lui.

Italian usually drops subject pronouns, so why is lui required here?

You cannot drop the subject pronoun when it is modified by a word like anche. To clarify who exactly is being included, you must explicitly state lui alongside anche so the sentence does not feel incomplete.