There's an entire beginners section on EverythingDulcimer.com. The site covers all forms of dulcimer, hammered, bowed and Mountain.
A waist strap's very useful, although some people play it cradled.
A few plecs and a slip-in on the side to house the one he settles on
Something to harden the left hand - fingering thin metal strings...
A side-gripping capo with single string stops.