Ralph pretty much uses rolls, instead of the alternating pattern (TITMTITM) Scruggs often uses. As for which digit plays the melody note, it depends on where the note is, both on the strings and within the measure. You can play a forward roll (TIMTIMTM) starting with the 3d string, 4th string, or even 5th, and a modified forward roll (TMTIMTIM) allows for the 2d string to be the thumb note. You can also play the melody note with the Index or Middle finger, too, or employ backward (MITMITMT or MTIMTIMT or variations thereon) or inside-out rolls (TIMITIM or similar variations).
And, being bluegrass, who needs melody, anyway? The melody is "in there somewhere" among all the notes of the rolls. Listen to Ralph's playing and you'll see that he has his cliches, but also his inventions, and it all fits together.
Bob (the-older-timier-the-better) Clayton