I don't think explosive anger is typical for SPD. The emotionlessness would be, but it sounds as though maybe your friend isn't truly emotionless, but instead shuts himself down until he explodes. The holding grudges thing also doesn't sound like SPD. I don't hold grudges because doing that takes emotional energy, and I just don't care. I avoid people that are mean but I do not ever attack.

It's a tough call, obviously. And if he is unlikely to seek help, you probably won't know what a diagnosis might be. I don't know much about sociopaths, but I also think they may not be inclined to explosive anger. From what I know, they don't act out because they are angry at the person or animal. They act out because they can. They don't tend to hurt people because that particular person has made them angry.