The jams are what's gonna get somebody interested in Phish. I've struggled to get friends into them and everyone had a hard time getting over their lyrics and singing.
I think "My Soul" is a good choice since people are familiar with blues guitar jams a la Clapton and Stevie Ray Vaughn. Also try these songs: Waste (Bittersweet Motel), Sample in a Jar (Album), Frankenstein, Cars Truck & Buses, Piano outtro from Squirming Coil.
If you can't get him to like Phish, try Widespread Panic since they are slightly more straightforward.