Let this child go NOW! You, your son, and any other children that come into your home are at risk of getting hurt. And if I was a parent interviewing with you to watch my kids, I would leave the second I saw or heard of this kind of behaviour. It's not fair to you or to your son to keep dcb around.
Kudos for keeping him this long. But honestly, we can't fix them all. Some of them we just have to let go.
As far as money goes, and saving for a vacation -- why not do something simpler and less expensive with your son? We go camping and do day trips to save money, and we pack our own food (usually rolls & cold cuts from the grocery store). As a result, our kids have seen more of our local attractions than alot of their friends
I know that one day, this dc boy will realize what he put you through and will be grateful that you stuck with him this long. But he'll also understand why you had to let him go. And who knows, maybe in a center he'll be able to get the help he needs.
Good luck and be strong!