Reminds me of every 944 I ever owned.
I'd tackle the t-stat first (easy and cheap- and gives opportunity to bleed system), but it makes me wonder why someone would replace a radiator and NOT change the thermostat in the process. To me, a PO would have had overheating, changed the t-stat first, then with no luck changed the radiator in frustration (unless radiator changed due to front end accident).
A quick internet search on 94 toyota camry overheating makes mention of plastic impellers on the water pump failing. There was some disagreement on if that car had a plastic vs metal impeller, but that would also explain the heater core not working. At least one post recommended checking impeller blades to see if they spun freely of the water pump while in changing t-stat (if that is possible).
One question is if the car has always overheated since purchase, versus this being a new thing for your son. I'm almost wondering if there is a very slow leak which allowed for a slow accumulation of air in the system, which vexed the PO's and caused them to eventually give up and sell the sucker.
Finally, a google search for "94 toyota camry bleeding coolant" pulled up some hits with some obligate youtube videos and cool music. It looks like a fairly easy system to bleed compared to some cars.
Good luck