I am going to sort of diverge from what others have recommended, and give you a couple of pieces that would introduce your father to new things (from your description, it seems your father has not tried a lot of new things, and has instead stayed within the realm of known, almost safe, composers).
The Rite of Spring by Stravinsky
Amazon.com: Stravinsky: The Rite of Spring; The Firebird Suite (1919): Sergey Prokofiev, Igor Stravinsky, Leonard Bernstein, London Symphony Orchestra, New York Philharmonic: Music
It is a very odd piece in that it does not conform to any standard rules of composition. I like to describe it as raw emotionalism, with very controversial rhythms and usage of instruments/harmonies. An example is that at one point a bassoon (or oboe, I forget which) does what can only be described as quacking. It has a very odd structure, and is very hard to listen to, but for me portrays a very special bouquet of emotions. The one I linked to also has the Firebird Suite and Prokofiev's Scythian Suite, both of which follow the sort of non-normal composition style. The works by Stravinsky are ballets, fyi.
I also second the Tchaikovsky Piano Concerto 1
Shostakovich 10
One of his best, it is also very historical. Shostakovitch was censored by Stalin in 1948 for not producing music that adhered to the Soviet Realism that the party demanded. This caused Shostakovitch to vow never to write another symphony while the dictator lived. When he heard news of Stalin's death, he vigorously went at composing this symphony, releasing all feelings and creative impulses that he had held in for fear of his life. It is a pivotal piece in Soviet music, and in a way reflects on Stalin as a ruthless dictator who murdered his own people and also on what he did to and what was to happen to the country as a whole. For a sort of compliment to this, I would suggest Shostakovitch 5, which was written after his first denunciation. It is artfully crafted, as it had to fit the heroic structure that the Soviet leaders wanted, yet had to not be a sellout to the regime. The end result was a perfectly made balancing act that the leaders saw as heroic, but that to the people was very clearly a statement of their despair and sadness under the oppressive Stalinist government.
R Strauss: Ein Heldenleben, a symphonic poem
Amazon.com: Richard Strauss: Also sprach Zarathustra; Ein Heldenleben: Richard [1] Strauss, Fritz Reiner, Chicago Symphony Orchestra, John Weicher: Music
I like the version by the Pittsburgh Symphony and Manfred Honeck, but that is not online as far as I can tell. It is a symphonic poem, as goes from very lyrical and sweet to almost forceful. It is just plain exciting to listen to.
Hope that helps,
Nkk (who, if you did not notice, very much likes Russian classical)
EDIT: also,
http://www.amazon.com/Symphony-11-Ye...9897924&sr=1-1
Read the description there, as I feel this is also a very interesting piece to listen to.