If you are only claiming ftb, it will come down to what the ato deem as income

I'm assuming he would be deemed as self employed, and would essentially have income of x amount (rent) and then deductions. So I'm with div - definitely check what the implications are financially for you guys. He may find himself working two full days and being out of pocket half the amount you would have paid in rent on "expenses" - possibly even more.

If he is interested in contract work he is probably better off with more transparency - he gets paid x amount and then you pay your rent as normal. It will be much more clear cut that way.