I would suggest the following pop/rock artists for your brother:
Jennifer Warnes "Famous Blue Raincoat" (20th anniversary edition) or "The Well" -- great voice and if he likes Joni he will like Jennifer
Christy Baron "Steppin'" -- another great female vocalist and a beautiful Chesky recording
Counting Crows, "August and Everything Else" -- kind of folky rock, and a great collection of songs
Peter Gabriel, "Secret World Live" -- amazing songs and performances (the DVD is probably even better)
Pearl Jam, "Live at Benaroya Hall" -- an acoustic concert fundraiser for a homeless youth program in their home town of Seattle, this show is packed with wonderful music -- rockin' but still musical.
Plus a bonus pick:
Paul Simon, "Graceland" -- OK, kind of cheat because he has surely heard of Paul Simon, but this is one of the great albums of all time, IMHO. The latest remastered version includes a couple of very cool alternative takes.
Happy listening!