To find instructional sheet music, you probably need to look for music made for banjo mandolins.
As far as the price for stringing, tuning and repairing the metal band, $150 doesn't sound bad. Tightening or loosening the drum can make a difference in the sound; it needs to be taut, but not overly. I recently had my 1907 Whyte Laydie #7 repaired with a new drum head, new strings, tuning, etc. It cost about $300. The drum head was not cheap because I wanted vintage to match the era of the banjo.
Is he replacing that band with new parts or vintage parts? Vintage parts are more expensive. A set of strings is probably about $10 or less. Tuning it takes about 5 minutes. It's the repair of the metal that is your most expensive. You should ask him how much of that $150 is labor costs.
Mother of pearl has many colors in it, as does abalone. Bakelite will look more like plastic (think 60's bracelets and purses made in bakelite).
Sounds like a fun instrument for your son to learn.