Huge good luck to him - DD is up for the same one, but too young to get anything more than a 'see you in 6 months' email so not too stressful for us.
They try to make it as fun as possible and if your DS can enjoy the process then that makes it easier.
Early rounds are pretty brutal - they will get everyone to sing and do their poem/monologue (do not worry about being cut short - it's not a bad sign) and will then call out the names of those they want to recall. Normally this is between 2-5 out of the 20 or so they have in a group)
First recall round is more of the same - but with a group of 20 who have made it past first cut. They then make another cut - generally those who are too young/too small are let go at this point and some will get a thanks, but no thanks email, and others will get a 'see you next season' email.
Those that are through will do a workshop session - and then there will be another cut.
These processes are very long - the most we have done so far is 7 rounds for one role, but I know people who have done 18 recalls!
Luckily DD loves the audition process - I just feel icky, especially if you get into final round territory.
Been on the merry-go-round for a couple of years now, so feel free to PM if you need any advice - lots of kind people held my hand and showed me the ropes when I was very green!