Future partner from Australia

I am an Australian who has been working in China for 10 years - my future partner is coming to visit and stay with me until my contract finishes in 18 months. He will travel backwards and forwards to Australia so we are not looking for long term - 2 - 3 months at a time. What is the best sort of visa - double entry or S2.
If S2 - private reasons - could we say to spend time with future partner??