Cdg to Gare de Austerlitz

Take a Taxi from the officials stand. It costs about 55 euros.
Where is he going from Paris?
It may not be a need to rush depending on the type of train he is taking.

There is no guarantee of his making his train, so I hope he can remain flexible. But to answer you question, yes, take a taxi. It will take the 3 of them to Austerlitz station for 55 EUR fixed price. Be sure to go through the official taxi line, and, so there is no confusion, have it handwritten on a piece of paper for the driver, "Gare d'Austerlitz, 55 EUR".

I say that because his flight could be late landing, long lines at the passport inspection point, heavy traffic into Paris. I don't know if they want to use the RER and Metro, as it would require a connection at Gare du Nord.

You did not ask but 3 hours is cutting it close -- the usual advice is to allow four hours between ETA at CDG and departure time from a Paris train station. What day of the week and what time of day is he arriving? In any event, he should just take a regular taxi from the official line. Taxis can use bus lanes and that gives them an advantage over private cars.
As mentioned before, there may be trains from CDG to his destination.

Regarding the above comment, there are no direct trains from CDG to anywhere on the mainline from Austerlitz. And CDG is poorly located for trips south by road (you have to drive halfway around the Paris region, which is stressful).
So you have to go to Gare d'Austerlitz, and a taxi is really best for that trip.
Also, I don't know what your final destination is, but trains towards Orléans, Blois, Amboise run really late on most days (last departure 10.30 PM or so), and advance booking is not essential, so your son can keep some flexibility.
en.oui.sncf is now much improved for checking train times and booking; there's also www.bahn.de/en (times only) and www.loco2.com (times and booking)

Thank you all so much. This has been very helpful.
Amy