Judging by what you say your Dad likes, here are some ideas:
Just about anything by The Band. Some choices include: The Night They Drove Ol’ Dixie Down, Up on Cripple Creek, The Weight.
If he likes CSNY, he might like some of their solo stuff. Stephen Stills did a good song called “love the one your with.” Neil Young has some good songs, like “Cinnamon Girl” “Ohio” or “Sugar Mountain.” Steven Stills was also in a band called Buffalo Springfield. Try “For What It’s Worth” Stills also was in a band called Manassas. Try:Fallen Eagle, So Begins the Task, Johnny’s Garden
Do you have just solo Clapton? He was in some bands: Cream (“White Room” “Badge” "Crossroads) Blind Faith (“Can’t Find my way home”), Delaney and Bonnie, Derek and the Dominoes (Layla).
Try some Allman Brothers, too. “Melissa,” “Midnight Rider” “Blue Sky” “Whipping Post.”
Some other choices:
Traffic: Low Spark of High Heeled Boys, Dear Mr. Fantasy, Feelin’ Alright
Santana: Black Magic Woman
Jethro Tull: Thick as a Brick, Song for Jeffrey, Fat Man.
T-Rex: Bang a Gong, Jeepster
Bob Dylan: Tangled Up in Blue, Lay Lady Lay
Moody Blues: Nights in White Satin, Legend of a Mind, Ride My See Saw.
All of these count as “Classic Rock” If you want something more obscure, (i.e., they don’t play on the radio a lot, but are still good) try these:
“Peace Frog” by the Doors. Familiar band with an unknown classic song.
“Black Magic Woman” by Peter Green’s Fleetwood Mac. This is the original version of the song Santana made popular. They also did a great instrumental called “Albatross.” Very mellow.
“Woodstock” by Joni Mitchell. This is a song that CSNY made popular, but was originally done by Joni Mitchell. I think this version is better.
“40,000 Headman (Roamin Thru the Gloamin With…)” by Traffic
“If”, “Summer '68” by Pink Floyd
“Hot Burrito #1” by Flying Burrito Brothers. This is slightly country sounding.
“Chestnut Mare” , “Ballad of the Easy Rider” by the Byrds. These are also vaguely country-ish.
“ooh la la” Small Faces. Rod Stewart had a version of this recently.
You didn’t mention the Beatles, or Stones, so I didn’t include any of that stuff, but it’s really essential for the time period.
Hope this helps some.