What amount of speech and charisma do you need to defeat him?

Speaking from personal Experience i'm not sure if having a certain amount of speech/charisma can talk him down but don't quote me on that, again you can choose to side with him, kill him (you may even be able to set the nukes off and run depending on your science skill) or if you talk to Vree in the brotherhood of steel she can give you a holodisk that will let's say change his mind.

You can also convince him to self destruct by providing evidence that his plan won't work. I don't think speech plays into that though. I could be wrong, I haven't done that method in quite some time.

ok thanks guys ive gotten the holotape and it should hopefully help.