Or should he just assimilate into Gentile society?
When I say "remain Jewish" I mean continue to do all the positive things he has done to live as a Jew up until his decision to believe in Jesus. And when I say "believe in Jesus," I mean believing something close to traditional Christian belief: Jesus is the Messiah and the Son of God, who died and rose from the dead, ascended into Heaven, and will return to judge the living and the dead.
Your respectful thoughts would be appreciated. Or better yet, you can offer your answer to my question here.
An example of an orthodox rabbi who tried to remain Jewish while believing in Jesus:
Rabbi Daniel Zion.