Morrillo is a colloquial term in Mexican Spanish, often used to refer to a young boy or kid. It's a more informal and affectionate way of saying 'niño' or 'joven'.
In the context of the song, the lyric "y el morrillo me tocó sin cables" (and the kid played for me without cables) suggests a young, perhaps self-taught musician or someone who is naturally talented, playing music in a raw, unplugged style. It adds a touch of authenticity and youthfulness to the narrative.
This corrido, "De Seattle A San Diego," introduces us to Luis R Conriquez, who proudly states, "yo soy compañero seré con riqueza" [I am a companion, I will be with wealth]. He's all about staying true to his roots, saying, "es del barrio de donde vengo no lo niego" [it's the neighborhood where I come from, I don't deny it]. He talks about working hard and always keeping his mother in mind, even though she didn't want him to leave school for his business ventures.
He's very clear about loyalty, noting that "amistades pocas personas son las leales" [friendships, few people are loyal] and that "hay muchos traicioneros y esos suelen disfrazarse de amistad" [there are many traitors and they usually disguise themselves as friendship]. Despite the fakes, he's firm with those who are genuine, saying, "soy firme con el que este aspirante" [I am firm with whoever is aspiring]. He's happy with his life and proud of his origins, declaring, "sangre estas pastillas con orgullo y sigo siendo el mismo de antes" [I take these pills with pride and I'm still the same as before]. He also mentions his style, from "zapatitos" [little shoes] to "deportivos" [sports shoes], and how he's a friend to his friends but not a show-off.