Hello dear friend, Deli!!! Its nice to see you posting again, I've missed you.
#1: I think it is AWESOME that you are at a place where you feel like you can disclose more about your past. That shows tremendous progress. So, kudos to you.
# 2: The only thing that worries me about this, is what is your intention in telling him? Why are you afraid you will lose him as a friend, just because he has a girlfriend now? I don't want you to feel like you need to disclose this big thing just because you think it will seal some emotional bond. If he is a good friend, he should still be one, even with a gf/moving out of college.
#3: If you want to tell him just because you have felt like you've reached a place of safety and trust with him, and it feels like a natural, next thing to do, that is a little different. Personally, I don't see why you need to explain yourself for what happened with him. That should NOT be a reason to disclose.
#4: I worry that you being home (grrr!!! but at least your family isn't there) is intensifying all of your worry for losing a friend (and is this rational/logical?), which is propelling you to disclose.
#5: From what I remember, he does have inklings that your family are a bunch of s***bags, and seemed compassionate. I wouldn't worry about it being "too much," because if he is a good friend, this will only help him understand you better, and hopefully bring more trust in the relationship.

I just hope you really think about WHY you want to do this, because it isn't seemingly as "straightforward" as telling your best girl friend. There are romantic feelings involved, even if it was awhile ago. That could get sticky, but also, you should only want to tell him because that feels right to you. Because its sort of the next step in your friendship, and you feel comfortable enough to know he isn't going to go running for the hills. Like I said, true friends wouldn't do that.

If a friend told me about a past like yours, I probably would have known something was wrong, because you can't hide things that big. But, I know the details would be what I didn't know, and a good friend should just listen. Let you talk, get it out, and listen. Probably get VERY angry for you. I would cry and hug you and never want to let you go. Thats just me

I am rambling, and not helping much am I? I guess I want to make sure you really truly think about the reasons behind this. If it feels right to you, deep down, then try it. Maybe not while you are at your parents house, because you are being triggered and in a different emotional state than you are at school. Have you talked to Pdoc or austin-T about this? If you can wait, I would see what they say too.

Either way, we are ALWAYS here for you.