The likes of Soft Machine, Magma, Can, Matching Mole, Gong and National Health should appeal to him.

Also, more free-form improv stuff like AMM?

agreed definitely!

Firstly, That list gives me a massive erection.

Secondly, I'd start with Can, and Ege Bamyasi. Firstly, like Yes, and Rush it's really accessible. But it's much more variant, and easy-going than both. Plus, it might introduce him to Soft Machine's Second, and Third. Second is quite wonderful, surprisingly poppy, beautiful piano centered rock music, and Third is a fantastic example of excellent Jazz fusion.

If he can get into that, then I'd bring in Magma which may be some of the most dense, and overpowering music ever written by human(and kobaian) hands. Maybe introduce him to 1001 centigrades, which has a 28 minute opening track that throws everything, and the kitchen sink out. Ideal for any fan of prog.

Thirdly, AMM will probably get on your nerves more, but is amazing.

Psychedelic Rock Recommendations
 

Hi!
I really like Kate Bush, does anyone have recommendations of artists that sound like her? Thanks!

^^Tori Amos?

Anything by Pink Floyd and drug era Beatles.

Cool! Thanks, Ill try those! :)

Cinder Cone
 

im loving Cinder Cone right now...they are a LA based band, who recently played a show with a band i dig called LIGHTSYSTEM...def worth checking out!

oh yes, annnnndddd.....Junius record, "Tales From The Threshold Of Death"...record of the month for sure!!!

Chimp Spanner
 

Give Chimp Spanner a try, I think your reaction will be extreme, one way or the other.