“Can he make it retro-active?”
I don’t see why not.
Somehow I don’t think it would be retroactive, once citizenship is granted it must be taken away legally. Not through an executive order. Congress could pass something, but I suspect they would not. It would be a huge upheaval in society, there are so many. I would love to be proved wrong on this.
The Constitution expressly forbids the making of ex post facto laws. That’s why not. You can change the law going forward but never backward.