I think you have a deeply unhappy little boy, and he is a little boy, only 8. it does sound as though perhaps you expect, and treat, him as a much older child. 8 is still pretty young, especially for a boy.
I think if he has been offered lots of things to do, and has rejected them, perhaps he actually needs more individual attention rather than being sent away to boarding school where, no doubt, they will discipline him, if that is really what you want.
You say, deep down, he is a nice, loving little boy, and I don't doubt he is. No child is born 'evil' or whatever.
As Dittany pointed out earlier in the thread, referring to another poster, 'priviledged upbringing' means different things to different people. Being offered endless activities doesn't necessarily = happiness. He might be perfectly happy just mucking about at home doing nothing in particular. I wonder if perhaps he was a bit spoilt and then distressed once you had other children and all the attention taken away from him??
Some of his behaviour can be explained as normal childhood behaviour in an 8 year old. My DCs (now mainly grown up) have broken windows on occasion etc but smearing excretement doesn't sound normal, I admit, although could be a one off 'silly' turn.
He may have Aspergers, that is a possibility, in which case it will be difficult to 'help' him in the conventional ways and you will need professional advice.
Please don't write him off as a 'bad' boy. He may be anything but.