As a starting point use kbb trade in values and add 30cents on the dollar for accessories IF you like them. Private retail is usually about 500-1000 over trade-in. Dont pay extra for stuff you wouldn't install. What he owes and what it's worth are 2 completely different numbers. Buy any vehicle without a large down payment and you end up under on the loan.
The 2018 are out and some dealers might still have 2015s. I'd negotiate a bit more. Does he have service records?
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2015 VSTROM 650 X ABS EXP
Previous rides: 2012 KLX250S, 2006 VT1100C2, 2002 KX125, 1999 SV650, 1987 GPZ305
Bikes I wish I had: Ducati 916, Harley Road King, KTM 500 EXC, BMW R1200RT, Honda CRF1000L, BMW R75