How you want to do this is also very dangerous.
Think for example if that GF leaves your step son, it's going to be costly for everyone in terms of stamp duty etc, plus having to buy her quarter out etc.
As for trusts, id be personally leaving them out of this arrangement. They're only good in a scenario of one person purchasing a house if they are setup correctly. 95% of them out there are setup so that others (such as a defacto) can stake a claim at the property if they leave the relationship, even if the property was purchased before hand.
Teach the step son to save that money himself and let him buy his own property. I did some splitting up of loan repayments etc and while people tell you it works, in the long run the numbers don't add up a lot of the time...