My experience of the entrance tests are that they are not that difficult, at least to meet the school standards. Meeting tagged course standards is harder (ones where the airlines "choose" the students).
The main filter for applicant appears to be the ability to come up with the money. So if someone fails the tests, either they clearly don't have the money or they're just not good enough, yet. If they had you back for a second go, it's probably because they realised you had the money. Find out what your son failed on and try to develop that. There is no point in spending your money if he doesn't have the right stuff to get through. Especially when it comes to the last part of trying to get a job.
I think some of the tests are nonsense anyway, but some of the assessment is relevant. They don't want to have to work too hard to get their students passed. If you feel that CTC got it wrong, you are better off looking at full time modular. Try somewhere like BCFT where you can do full time modular for less money with good instruction.
If you still want to go with ACA, it's your money. Funny enough I recommended a friend against paying for his son to go to PTC, just before they went bust. He didn't listen and they lost a lot of money. Probably won't happen with ACA but you won't get what's sold on the tin.