Do u wanna express that he writes charaters simply as a Chinese does? 简直 = simply it is a adv. which should put it after a verb. the grammer of Chinese obey the rule like: S + V + adv.+O

他写汉字简直，像中国人一样。 他简直写汉字，像中国人一样。 In fact if you say"他写汉字简直像中国人一样。" or"他写汉字，简直像中国人一样。" they are better... Because you want to say "the Chinese characters he write" not "he"...

i think the first is right. and the second one,we don't use like that.

the first one is right,the sentence means a man write Chinese characters very well,as if the native Chinese

Remove the comma