thats very general...could be the battery, or the throttle body, an O2 sensor, bad gas, leak in the intake/exhaust, water in the engine, bad seals, carbon build up, etc.
Best bet is run the car for a bit (few mins or longer), take it to WOT (redline) if it warms up then see if it fixes itself.
Edit: I see some of the other people have chimed in with part-replacement suggestions. Try running the car first, see if you can get your son to spot any weird things happening. No need to jump to replacing parts without a firm diagnosis on what the problem is, especially in a car that old.
It depends does it do it all the time? I had the same problem in my ford bronco and turns out it was a cracked block.. my guess would be start with the basics oil change spark plugs fresh gas check the gas lines makeshure its not leaking and def. Check the idling if its idling 2 low it will cause it to clunk out if u take the gas off. If u don't no how 2 do this take it to a local auto zone or advanced auto they can change the idling for u and if this doesn't work they can run a test for free u just gotta ask