I would suggest that your son is mistaken, Grateful Dead, ARE DEADHEAD MUSIC,, or imo.Jazz, Long musical improvisation normally comes under the category of Jazz
wiki describes them thus.
The Grateful Dead were an American rock band formed in 1965 in the San Francisco Bay Area.[1] The band was known for its unique and eclectic style, which fused elements of rock, folk, bluegrass, blues, reggae, country, improvisational jazz, psychedelia, and space rock[2][3]—and for live performances of long musical improvisation.