Oh man is he in luck.  Your son-in-law can put together a *wonderful*
classical collection on Lp, addressing both depth and breadth.  There
is more good-quality classical music out there on Lp than from just about
any other genre--Philips, DG, most RCA Shaded Dogs, EMI/Angel.  Unfortunately
Columbia's and RCA's aren't always that good.  He will find also that
digital Lps sound very very good compared to their CD counterparts;
record pressing didn't really stop until '89.

Collector's items are just the tip of the iceburg and luckily not always
the best available performance!  The other rule is: the more famous the
recording, the worst the artistic merit, LOL, (but not always the case).
The best way to flesh out a collection is to visit big stores in big
cities.  Make a weekend out of it.  Most classical albums cost @$2 in
the big San Fransisco stores.  Ship them home, or carry them.  There are
tons more Classical Lps available on ebay, though when all is said and
done, you're looking at $7-$15 an Lp after shipping costs.  There are
many online used stores to check out as well.  Look in the back of
Stereophile and The Absolute Sound.

When my friends hear an Lp on my system, the first thing they ask is,
"where's all the clicking and popping and noise?" and "I didn't know
Lps could be so dynamic!"

Neither did I, but I'm glad I do now.  We may see vinyl make a comeback.
I've seen turntables in a couple of commercials, and also making cameo
appearances in recent movies.  As far as new Lps go, there's a lot of
classical available but they are new, audiophile heavy pressings of old
recordings--the collector's items you are talking about.  I know Tony
Faulkner just recorded John Lill performing some Schubert works which
was released on vinyl.  Maybe this is just the beginning.

John Smyth