I'm not sure about how the calculations will impact you but I do know you can get some accounts without bank fees. Some of the direct saver accounts which are 100% online have no bank fees, though you will accrue interest which you'll have to pay tax on (though it probably won't be much). Do you have to have CSA? If you need it, fine, but if you don't maybe it will be easier if it's not paid rather than have to pay it back. Can Centrelink help at all?

I assume the ATO know he's overdue with his tax returns? Otherwise I'd suggest dobbing him in. Some people are crap with doing tax returns. I had an ex who was so far behind as he owed tax and CS and thought avoiding it would punish his DD's ex.