Jayne

It's definitely time to have that talk whether he sees this friend or not.
I taught in public schools for 32 years and am currently the president of a local school board.  I do know that there are rare places across the United States where ungodly things are taught, but it's not rampant among public schools.  We aren't the devil like some are taught we are, but that's another thread.
You can just simply tell him that sometimes these days there are young people who want to live as the other gender.  They change their names, their clothes, and sometimes their bodies.  But that really doesn't change whether they are a boy or a girl.  Tell him that they are born and boy or girl and that's how God made them no matter how much they try to believe they are changed.  This is would be an ideal time to discuss the Biblical truth that the devil is the Father of Lies and how his lies can really, really harm people and lead them down a path of shame, sin, pain, and away from God.
Tell him that this confusion over who they are causes them much pain in their hearts and minds and many of these young people are hurting.  They need prayer.  You can tell him about his friend and pray with him for his friend.  
One thing I can tell you for certain.  Even at 8, he knows more than you think he knows and had been exposed - yes even as a homeschooler - to more than you think he has.  Do  you own a television?  Even commercials for ALL programs contain questionable material.  Have you been present at every conversation he has had with every friend?
Conversations like these are not easy.  But they are vital.  And since he now knows someone who is transgender [even though you haven't told him], now would be an ideal time to have that simple conversation in terms of scripture and God's purpose for humanity.

Both the KJV and the NIV combine verses 12 and 13 into one verse.  I'm sure the majority of the other English translations do too.
King James Version verse 12 (includes LV 13): = "Kiss the Son, lest he be angry, and ye perish from the way, when his wrath is kindled but a little. Blessed are all they that put their trust in him."
NIV verse 12 (includes LV 13): = " Kiss his son, or he will be angry and your way will lead to your destruction, for his wrath can flare up in a moment. Blessed are all who take refuge in him."
edited to add:
There are going to be numeration differences - especially in the Psalms - when you go from Hebrew to Greek Septuagint/Latin Vulgate to English to Spanish ..... they just numbered them differently
But, the content is the same.