If your son is happy with the Ric strings then you can order direct from Rickenbacker:
http://boutique.rickenbacker.com/Strings_c_14.html
Otherwise, there's no law that says you have to use the exact same gauge or type strings. Find something close. You may have to adjust intonation at the bridge (adjust the saddles), but that's a task that inevitably will need to be learned anyway.
^^
AXES: Fender '81 The STRAT, '12 Standard Tele, '78 Musicmaster Bass, '13 CN-240SCE Thinline; Rickenbacker '82 360-12BWB; Epiphone '05 Casino, '08 John Lennon EJ-160E; Guild '70 D-40NT; Ovation '99 Celebrity CS-257; Yamaha '96 FG411CE-12; Washburn '05 M6SW Mando, '08 Oscar Schmidt OU250Bell Uke; Johnson '96 JR-200-SB Squareneck Reso; Hofner '07 Icon B-Bass; Ibanez '12 AR-325. AMPS: Tech 21 Trademark 10; Peavey ValveKing Royal 8; Fender Acoustonic 90, Passport Mini, Mini Tonemaster; Marshall MS-2 Micro Stack; Behringer BX-108 Thunderbird; Tom Scholz Rockman. PEDALS/FX: Boss ME-50; Yamaha EMP100; Stage DE-1; Samson C-Com 16 L.R. Baggs ParaAcoustic D.I; MXR EQ-10.