No he does not need to. RCM under section 9(4) CGST Act & 5(4) IGST Act are liable only to notified list of persons who would be liable to such RCM. Presently, no such list has been notified, which means, No RCM liability for purchases from unregistered person.
No you cannot take ITC Concept of ITC Comes only when you paid GST on purchases... Since you are purchasing from Unregistered Dealer so you are not paying any GST at all... hence the question of getting ITC not at all arises.