Okay, so it turns out the way someone had mentioned. He thought he was doing me a favour since my horse hadn't been ridden in a few days. He told me that the girl who was riding him, is also a private boarder. He asked her not to ride her horse that day and if she wouldn't mind riding mine (since I'm sick). He also told me she is upset with him because of what happened. I told him I really don't care how she feels or whether she is upset with you or not. Fact is I do not want my horse ridden without my permission. Anyway, he said he made a big mistake, he really thought he was doing a good thing and now he has two boarders upset with him. He also promised this was the first time, and only his son had previously ridden Corman. He also apologised for the cut under his forelock and swore he hadn't seen it.
Some might think me naive, but I believe him. As I first mentioned he is a good man and he has helped me a lot over the last few months, so I'm going to give him the benefit of the doubt and stay on as a boarder. However this doesn't mean I've put my guard down. Both my fiance and I will show up there at random hours and make sure everything is as it should be. Also today I will be buying a tack box so my saddle and bridle aren't accessible. If anything of the sort happens again he will be moved immediately.
I want to thank everyone for their responses and thoughts. I really appreciated all of them.