RE: Why has he started refusing jumps?
i would check him out inch by inch!
every part of his legs, neck, back for any hint of pain, strain or injury. he is an older horse now and things will slowly be catching up on him so needs to be treated as such. consider adding oil supplements etc for his increasing age, it will help him now but also for the future.
check the same with his hooves, all areas for pain or pressure, and ask the farrier to do a pinch test when he is out next.
also check your saddle still fits correctly or may need re-padding or even changing.
if totally sound and pain free, i would go back to some basics, trotting poles, cavaletti, mini cross poles etc, all nice and easy and regain his confidence. but id still review his diet and supplements for his age.