You will want to google for horology and horological plans and drawings.

Here's some (not all plans, and not all free):

http://www.wrsmithclocks.com/books.htm

http://users.tpg.com.au/pgc123/Home/horologicalplans.html

http://www.lautard.com/clockpla.htm

http://infomotions.com/etexts/gutenberg/dirs/1/7/0/2/17021/17021.htm

He might enjoy designing his own escapement:

http://www.geocities.com/mvhw/TToc.htm

There's lots of room for guesswork, actually. It's only the oscillator that keeps time. Sure, it's precise work, but there's hundreds of different ways to do the escapement and gear train. Building a bunch of different escapements would be a fun project.

I have a good friend who is an accomplished amateur clock maker who designed and built his own precision clock quite a few years ago. He's working on a new all-brass clock with a perpetual-calendar complication of his own design and a roughly 2-meter pendulum. He built himself a brass foundry so he can cast the blanks and then restored a 100-year-old gear hob so he can cut them. It's quite an impressive project. He has an old textbook he swears by; I'll ask him the title and author as I've forgotten.

Jim