My first instinct is, when a client doesn't know what Github or something like that is, just use it and teach them later if you must. For issues like this that have relatively limited downside, better to ask forgiveness than permission and just get on with it.

Regarding his response, that usually means call him and explain it in a conversation. Hackers tend to be email-oriented b/c it's asynchronous, but many business people tend to be phone-oriented.

Short terse emails like that where they don't elaborate on why they don't understand or ask any followup questions tend to be a strong signal that they're not email people. So just schedule a call and explain instead.