very common scam in Bangkok, they are a salesman/tout.  They always have a relative in australia.    Never heard about the lunch invite, but I am sure this conversation was going to end up you being led to a shop, for overpriced goods, which they will get a commission.  If you try to say you want to go somewhere else,  they will tell you its closed for government function or something like that.
I have lost count how many times they tried to hit me up with this one.  Its amazing that I was 20 years between ttrips to Thailand and the same scam continues,  word for word.
The biggest clue to any scam overseas.  if someone approaches you to strike up a conversation under pretence of idle chat, practicing english,    its a scam.   Normal people dont do that.