Gday Benno,
You will find that you will get a lot of different opinions with most people just recommending what they ride.
In order to get a better understanding of your sons requirements a bit more info would be helpful ...ie: where are you located ?
the average wind ranges vary so much that what may suit a 75kg rider in Melbourne would not suit a 75kg rider in Perth....ok a more stark difference would be Melbourne and Geraldton etc....
Let us know what state and or area your son will be riding and I am sure you will get some replies or at least get pointed in the right direction to a retailer or kite school in your area.
At a guess if you are in Sydney, Melbourne, Brisbane and most places in between a 10m Hybrid/Bow would probably be the best size for a 73kg rider to get started and make the most of it this summer. This size should see him comfortable from 14knots to about 21knots.... if the average winds are higher in your area perhaps am 8m would be better if they are lower a 12m ( although thats getting pretty big as a one kite quiver for 73kg person )
good luck I am sure he will be stoked