Kathie, that was a fear reaction, classic "I must protect my space" thing. Yes, it hurts, but he needs to have it explained to him (without getting angry, because fear will over-ride just about all carefully drilled social politenesses) that there was no threat intended to his possessions.
You go over it gently, maybe with easy child out of the room so he knows he is safe. You then explain that while he may have THOUGHT she was about to ask for his toy, she actually was making a polite, interested query with no strings attached. You then rehearse a more appropriate response with him.
If you can get to the next stage, do it. if not, leave it there. But the next stage is, you call easy child back in and ask her to repeat what she asked, in the same tone. Hopefully the rehearsal will work. If not - you tried.
These problems will keep happening, while he continues to be afraid for his possessions. The fears can be intense and unreasonable but they won't go away until he is reassured. And if you punish him by taking away possessions, this will make the fear worse because you have just given it reality. The best punishment is to get him to do it properly and apologise for hurting her feelings over polite conversation. Then you leave it and move on.
Every time you have a success, you help de-fuse the fear. And surely the ultimate aim is to equip him to interact appropriately, and if he needs to feel safe to do this, then so be it.
We can't raise our difficult children with the same rules we were raised - it just doesn't work for them.
Marg