S.F.
As a Kindergarten teacher, I have seen this before! Try the blackboard paint, which should make that wall more appealing to him (you could even have him 'help' paint, and maybe even paint a frame around it!) or you can go to office supply stores and find static cling wipeoff sheets. They stick to the wall with static and all you have to use is the washable dry erase markers.
Also, if you let him help paint the walls of his new room, he may take some ownership in it and not want to destroy it by writing on it.
GOOD LUCK!