It's music like Morricone does some of it's music, remember the intro sequence in Once upon a time in the West, with the bad guys waiting in the train station, the fly the screeching sound except it's not for a movie score.
It's easier to categorize by what it is not than what it is. I'd go with symphonical street music.
I have a compilation of him made in England that puts the LPs Moondog and Moondog II, gotta say it's great stuff.
http://www.cduniverse.com/search/xx/music/...FMoondog+II.htm