Not sure there is a lot you can do for it at this point. JM's are prone to a variety of soil pathogens that can severely damage or even kill the tree. This one looks like it might be dying a slow death :-((

What I would suggest is getting rid of all the rocks - they are not helping and could be hurting the tree. And carefully prune out all the dead wood you see. It will be a pale color, often gray, compared to the dark color of healthy tissue. Dead wood tends to harbor insects and allows an inroad for other disease issues. Then just keep the tree adequately watered - not too much, not too little. Just don't allow the soil to dry completely. And no fertilizer!!

If the tree does not respond or seems to get worse, then it's time to contemplate removal. But I would be cautious about replanting with another JM until you can confirm what caused the problem (and that may involve a path lab analysis), as some of these soil pathogens will infect any other susceptible species planted that space.

Going forward, you might want to post any other questions you have about Japanese maples on the Maples forum. This one is pretty much inactive and most JM enthusiasts post there.