My kid wants a used Mac Pro for Pro Tools...

[Speedy]

Get the new model, the ash can, when it is released in three to five years or so.

[rich in distress]

A good rule of thumb would be to adhere to his needs of today, I think.
That kind of tech changes very quick, and unless he makes a substantial investment in external hardware, it does not make sense to use old computers.
Today's native options are very competent, ProTools included.
For my first ProTools rig, I bought a Quadra 650. Time after that the 24mb RAM.
By the time i got the audiomedia2 soundcard, I did not want to run ProTools anymore so I went for Digital Performer.

[edgarbc1]

if he is using a USB or FW based interface then any Mac which can support the software version he is planning on using.

if he is planning on using Avid PCI cards for either current versions of Protools or older, it matters based on PCIe or other stuff which I never had time to learn the incompatibilities with older Mac Pros and the cards

i use PT11 using a USB interface and iLok usb dongle with a MBP 2011
without a hitch. i replaced the optical drive with another 750 gb HD drive for recording and the main OS drive is a replacement SSD HD.