I don't want my best friend to remain friends with my ex
Why wouldn't you want the two of them to remain friends?
Because I'm pissed off, and she's my ex.
If they've been trying to hide it from you then I understand why you'd be upset.  Friends shouldn't sneak around and do things behind eachothers backs, and you should speak to your friend about it.
Should I tell him that I'm angry that he's been hanging out with my ex?
I think telling him would be perfectly fine. He might not know you are upset about it if you don't tell him.