welp, the main problem with his hair is that you CAN'T CAN'T CAAAAN'T bleach a wig, so you'll probably need to start with a white wig.
Get a nice long white wig with bangs (Ivy, perhaps), and get a bunch of sharpies in a nice dark green and black. Bust them suckers open, mix the colors together with some rubbing alcohol in a spray bottle and spray it on the wig.
You could also just sharpie directly onto the fibres to keep it more where you want it. I'd avoid straight out dyeing since you could slip the ends in and you won't be able to undo it :\
Always rinse your wig thoroughly with cold water, otherwise you'll get sharpie on EVERYTHING.
Another way is white extensions sewn into the wig? His hair is a pain in the butt >_x