Pray for Susie Green and her entire foundation. If they do not change thier ways, hand in hand they all will be in the Molten Lavas of Hell!!!
This is what happens when the left controls the debate and the language and it is happening with disturbing regularity. 
Instead of using a pronoun at all, why not say “ Your formerly male son who has transformed into your now female daughter”?
Sounds dumb because it is dumb but it's accurate and that’s what society has come to of late. Furthermore, it points out the abject stupidity and the bizarre - contrary to nature and common sense - actions of the “trans” child and its brain dead mother. 
Suspending Pope Francis’ “dignity of the person” for a moment and risking arrest from the thought police, instead of a simple pronoun, I would prefer to refer to the creature as “a screwed up, freakazoid, son of a beyotch.” 