Hi S.,
I do hair. The answer to your question depends on the color of his hair. If his hair is a light color- darkest dark blond or very light brown, then you can get some color to stay for awhile. If he has dark hair, bleaching it will definately show his colors the way he is hoping to. You can decide when to stop the bleaching process (so it does not have to go to a planinum blond to be effective). Bleaching of the hair opens the cuticle and makes it easier to grab the chemical treated hair. You can allow the color on his natural color and get some result (again, depending on his natural color)- the black will show, but maybe not the green.)
Some things you may need to consider before going all out and allowing bleaching as part of his summer is-
How long is he hoping (or are you allowing)to have his hair green and black?
Do you plan on doing this yourself, or going somewhere?
Have you allowed for the full time it takes to (mostly)rinse out- then it will be back to it's bleached color- if you are lucky.
Are you okay with him going back to school with some of this still showing?
Do you know that you might have to do more chemical treatings to get it close to how you want it (removing green and black, and possibly coloring his hair back to his original color- which is not an easy thing- no guarentee(sp) of color matching- it will be frustrating), you may decide to cut some of it to make it look closer to the natural color if for some reason it hasn't faded in time.
There will be plenty of things to consider. Is he is willing to cut it super short afterwards to get it back to normal color, if necessary- if that is even very important to you- I am not sure.
Let me know if you have any questions and I will be happy to help you. Make sure you talk to SOMEONE who knows hair).
Have fun!!
A.