Without doubt the 8" dob, neither of the other options will show a great deal to someone of his age from the backyard (given light pollution) simply because they lack enough aperture although the 80mm Orion is not so bad from the burbs, its problem would be the complex EQ mount you would need to use it properly. The GOTO features of the Meade are actually somewhat of a disadvantage because they introduce a degree of complexity to something that should be for pleasure. Bottom line both of those options require a fair amount of work to set up plus patience. 8" dobs are fantastic workhorses that keep many enthusiasts in Astronomy for their entire lives. Learning to star hop is a skill that will last him his entire life as well. I joined an Astronomy club in my High school and waited months to be allowed to take the scope home the first time. No technology then so it was all star hopping followed by setting circles which really is a dead art. That was 30 years ago and I am still obsessed and still learning. The more glass you buy now the more he will see and appreciate and importantly it wont cost you another scope when he decides to look at something other than the Moon and Jupiter.
There is one other option you might wish to consider, an Edmund Astroscan. They are not sold in Australia so you would need to order it off the net but they are purpose built for kids, right height, right size field of view, right amount of glass and finally the right amount of skill requirements which means nil. They pop up in Australia second about once a year, I just bought one from the US second hand for $380.00 AUD including shipping.
http://www.dansdata.com/images/astro...troscan800.jpg
Paul Medcraft