Health club automatic payments after cancellation

[JayKay]

Quit health club (for my teen son) and told owner/personal trainer, well in advance and on day left . Found out this month he had been taking dues each month for 20 months. Owner said even though he knew of cancellation, had to be in writing (according to boilerplate contract). Asked for refund, refused, only offering membership credit for future use.
1) Is this not unjust enrichment?
2) Shouldn't judge make both parties whole (make owner re-pay; no question haven't used club).
3) Isn't contract unconscionable?
4) Would contra proferendum apply?
He believes my negligence (credit card hijacking not caught) means he can keep the $780. While not following contract to the letter, I acted as a reasonable person would. He never said to make sure to quit in writing on day we left; he could have had paperwork ready.