To add to the above post...
As stated above, your son should run the best available video from the sources directly to the TV. Usually, for the DVD it will bethe component output. This requires three video connections (red, blue,and green) from the DVD to the TV.
For the audio, the best sound will be obtained by running a digital connection from the DVD player directly to the receiver. This could be either an interconnect with an RCA plug on the ends, or a "toslink" cable, which has a square plastic connector on the ends. Given a choice, I prefer the former.
Additionally, if he wants to be able to use the DVD and the TV with the TV speakers, he can run a stereo interconnect (red/white pair) from the DVD player directly to the corrosponding audio input on the TV. He would turn the TV volume all the way down when he wants to use the receiver.
As for using the TV through the AVR, he needs to run another stereo interconnect (red/white pair) from the TV's red/white audio outputs* to a red/white input on the AVR.
In any case, he will select what he wants to watch (video) on the TV and what he wants to listen (audio) to via the AVR. When he listens through the AVR he should turn the TV sound all the way down.
* Check the TV's manual for more on this.