It depends a little on what he's planning to do in the way of repairs, and how his bike(s) are equipped. I do think that Park Tools are worth the money!

Some must-haves IMO:
good cable cutters
a chain tool
a cassette lockring and a chain whip
a nice set of T- or L- handled hex wrenches
the appropriate bottom bracket tool
a crankbolt extractor (if needed for your bike(s))
a pedal wrench
Headset wrenches (if needed for your bike(s))
Cone wrenches in the right sizes for your bike(s)
a work stand!

These are the tools that DH and I use over and over again. Your DH probably already has tire levers, patch kits, open end wrenches, etc., so I wouldn't bother spending money on them again.