2 points:
1. I would suggest your boyfriend have councilling or other supports lined up before he does this. From you statements, you don't expect the mother to handle this well. Your boyfriend can have these supports ready for the children as they adjust; I wouldn't trust her to have their best interests in mind. It sounds as if she's protecting her image of herself rather than the best interests of the children in this scenario.
2. I know the laws in Canada are different, but here she CANNOT interfere with your boyfriend's right to see his son. Or, more exactly, his son's right to know his father. The 8 year old also has a right to know his father - especially if that father has been trying to contact him in the first place. The only exceptions are where the children are at risk, otherwise she does not get to pull the dance strings. And if your boyfriend's concerned about not seeing the 8 year old (once he knows he's not the father), ours laws would extend the parental rights to him for the 8 year old too because he has known him as his father for x #of years. the basic biology isn't as important as the bond, relationship, and perspective of the child.
Good luck to you.
It is not so much a question of age as it is a question about a child's ability to understand what is being discussed. An eight-year-old is perfectly capable of grasping this type of information, and it would make sense to tell him. Then, those who are close to him can help him to deal with the reality. If the relationship between his mother and your friend were good, it would make sense for the two of them to tell him together. However, your note indicates that the climate is not favorable for this to occur. Thus, your friend, the person he regards as his father, should notify his ex-wife that he intends to tell him and then proceed to do this, even if his ex-wife disagrees. He needs to know the truth about his history.