This summary is drawn from the Rebbe's letters to him (about 5717–5721, 1957–1961); I did not find a published biography.
The correspondence is a sustained exchange on questions in Chassidus and Jewish thought.
Teshuvah and forgiveness. The Rebbe distinguished between immediate forgiveness, granted as soon as one sincerely asks, and "immediate redemption" as described in Tanya (IK 5842), and reconciled Tanya's teachings on teshuvah with Divine forgiveness: true teshuvah requires a sincere commitment not to repeat the wrong (IK 7386).
Gan Eden and the World to Come. He explained the levels of Gan Eden and the nature of angels according to various sources (IK 6300), the Rambam's view that some levels of Gan Eden are physical and some spiritual (IK 6357), and that Chassidus follows the Ramban: since body and soul perform mitzvos together, both receive their reward together (IK 6488).
Deeper concepts. He explained that "the Avos are the Merkavah" means both self-nullification before G-d and serving as a channel for G-dliness (IK 6168), and pointed him to further sources on G-d's knowledge in Shaar HaYichud VeHaEmunah (IK 6994).
Fixed times for Chassidus. The Rebbe urged him to set fixed times for studying Chassidus, and that such study should permeate one's whole being and daily life (IK 4937).
Sources: the Rebbe's letters cited above.