One of two scenarios come to mind and they're completely opposite. They're either idiots and don't know what they're doing, or they have been playing this game before and know what they're doing. Either way, to fight their over-charging it's still the same process.
Start documenting everything conversation you've had with him (or his daddy), in a diary is best. If you can remember word for word what you both said then direct quote it in your notes. Was anyone else there for those conversations? If yes get them to adopt/sign your notes to corroborate what was said.
Gather your evidence, photos before and after, if you don't have photos then take some. If you know any sparks that are willing to come and quote on the same work or provide a written report on work done and their estimation on cost then get it done. If not try and measure and count the materials he might have used and cost them out. There's not much you can't price on the net these days.
Jump on this website?
www.magistratescourt.wa.gov.au/C/civil_matters.aspx?uid=1226-6994-1097-9582 and start reading the fact sheets, Fact sheet 2 has a handy flow chart for you so you can read the ones that are relevant to you. I'd also read the ones relevant for him so you know what he (or his daddy) have to do.
No lawyers are involved so it will most likely come down to who is more organised on the day. Small Claims are heavily reliant on procedure, forget a form or don't lodge it at the right time and you start again. It's sounds like you've already been generous in your payment and their claim for another $2,000 is excessive. I'd fight it and call his bluff. If he's an idiot your preparation will scare him off, if he knows what he's doing them you'll need your evidence to fight him.