McMike .... Congrats on teaching your Son some music on his cello that can later be translated to upright bass !
Jake deVilliers had some good suggestions (like he always does).
Here are some other common jam (fiddle) tunes .... These are traditionally in D (only in 1 / 4 / 5 chords D/G/A) .... 8th of January, Flop-Ear Mule (only 2 chords D & A but usually there is a key change to A which means moving to A & E), Home Sweet Home, Liberty, Mississippi Sawyer, Panhandle Rag, Ragtime Annie, Wind Shakes The Barley .... and one your Son should definitely know is Saint Annes Reel.
If your Son wants a challenge in the Key of D .... Everyone loves Ashokan Farewell in D but beyond the 3 "normal" chords it also has Bm, C, and Em (on bass or cello we just play B, C, E major notes and leave the fancy minor-stuff to the fiddle and guitar). Arkansas Traveler is mostly 3 standard chords in D with an "E" once in awhile but it is a real "busy" tune that will keep your Son on his toes and ears to make the quick changes.
In A .... There's (traditionally) .... Bill Cheatham (quick chord changes), Black Mountain Rag, Road To Columbus, Salt Creek, June Apple, Kitchen Girl, Cluck Old Hen, and Red-Haired Boy.
In G (at least where I play bass) .... There's .... Big Sandy River, Big Sciota, Blackberry Blossum, Cripple Creek, Dixie Hoedown, Red Wing, Sandy River Belle, Seneca Square Dance, Colored Aristocracy, Temperance Reel, Kentucky Waltz, Turkey In The Straw, Golden Slippers, Grandfather's Clock, and Nail That Catfish To A Tree.
He should also learn Billy In The Low Ground in C. It is just 3 chords .... C, F, G but there's a lot of A-minor too. That tune comes up in jams fairly often.
I have played all of the above on bass and have chord charts. I will try to fish out my PDF's from PhotoBucket and post them. In the meantime, I highly recommend getting the Steve Kaufman's Bluegrass / Fiddle Tune Workout Series from Homespun .... He has the chord charts and CD's so you can get the tunes figgered out.
Good Luck, Have Fun, and tell your Son that Jams always need a bass player that knows the "standard" jam tunes. I went to a Montana Old Time Fiddlers Jam last week-end and I couldn't leave the stage for 4 hours. All the Women asked me to play with them (on bass) since I know most of the tunes / songs and I can trust me "ear" to help me through the ones I don't know.