I reckon you would get away with calling him Staffy x Kelpie and leave it at that breed wise.
And if anyone tells you something the least bit scary sounding - ask them how they know that? If they start with "they say", ask them who are "they", etc. Just keep asking more questions. And don't believe what you read in newspapers.
Every single newspaper story I've read about something that I was at, describes something completely different to what I remember.