No he can’t. He’s still a General after he resigns. Now if he resigns his commission and forfeit all his retirement then he will truly be free to speak.
Having said that, it’s unlikely he would be punished for speaking out as a retired general.
Would resigning his commission make him lose his retirement? You get paid for what you have done, not what you can do.
Just curious.
I know that Generals cannot really retire and are subject to recall.