Besides offering him Salvation, Exaltation and a personal relationship with Jesus, the Church can offer him some information about his lineage in many ways ;-)
He can go to the Church's geneology site here -http://www.familysearch.org/
Modern Saints can get their lineage through Revelation through a Patriarchal Blessing (like it happens in the Bible). Those who are not directly descended from one of the twelve tribes can be adopted in. (Much as Jesus' lineage is listed through his adoptive father Joseph). Most Native Americans and Mexicans I have met (who are Mormon) are identified as descended from the Tribe of Manasseh. Most Northern European descended Americans are the Tribe of Ephraim. The Tribe of Manasseh and Ephraim have specific Biblical blessings prophesy about their role in the last days. (On a side note I met a woman whose mom was Mexican and Dad Anglo. She was identified as the Tribe of Ephraim while her siblings were identified as the Tribe of Manasseh.)
Also, the book "House of Israel" by Hyrum Andrus lays out geneologies for different Tribes from genealogial records.
BTW, I met an Indian from the Tigua area on my mission who said his Tribe had religious teachings passed down about the White God who visited them, and taught them better farming and how to live in peace with the neighboring tribes. He painted me a picture of Chief Sitting Bull (Lakota) that I keep in my office.