When I returned the water smelled as if it had soured and my cherry bonsai leaves were yellow. I realize my tree was overwatered but what can I do to save him?
Let it dry out.
Seriously, have you ever re-potted it and what type of soil is it in now? Is it heavy and peaty like conventional potting soil or is it gritty and airy like small rocks and bark?
Gently remove it from the pot and see if you can determine the condition of the roots. An emergency re-potting may be in order. This will give you an opportunity to examine the roots and prune any dead material away. The problem is that, ideally, it should be in a gritty, free draining medium and you probably don't have any around. Have a quick look at this sticky.
You could also simply re-pot with whatever you have on hand or just cross your fingers and wait it out.