College age son rented clarinet for 3 months in 10/2013. He never paid rental and never returned clarinet

Thety sent matter to collection agency. Parent found out about it today, Called company and agreed to return clarinet and pay for the rental change from 10/13/2013 until now. They refused. Their website says I can return the instrument at any time. They are billiong my late and collection charges.totalling $1388 (total cost of buying the clarinet is $995) . Since their website says I can return the instrument at any time, should I go to court claiming that I should only be charged the rental charge to this point (even though my son was not responsive to their payment requests - he has no money - so you know who is stuck paying for this)