There's a lot of rappers who converted to Islam but I'm reading about some that converted to Judaism while in prison.
Yitzchak Jordan (Hebrew: יצחק יורדן; also known by his stage name Y-Love) is an orthodox Jewish rapper.
He used to occasionally attend a Baptist church when he was young, but later converted to Orthodox Judaism.
Born to black and Puerto Rican parents[1], he found understanding for his early stirrings of faith from his maternal grandmother, Clara Lopez. Named after the matriarch of a Jewish family that once gave her father a job, Lopez had served as a “Shabbos goy” as a child, and wasn’t shocked when Jordan donned a kippah to observe Passover. “As I started separating myself from the rest of the family, she acted as my interpreter,” recalled Jordan, in an interview with the Forward. Soon after that, as a child, he began to give his Jewish classmate his lunch money in exchange for teaching him Hebrew[2].
Doesn't say anything about prison here. In fact it sounds like he wanted to observe Judaism even as a child.
If they are really keeping the mitzvot, which it sounds like this guys is (you said he keeps Shabbath), then they are legit. Why do you question it?