I must say your son is MOST lucky to have parents who are so supportive and are willing to sacrifice so much to make his dream come true. I hope it does eventually.
All I would say is that it shouldn't be THAT hard. It speaks volumes about the state of the airline industry that families should have to sell cars and give up family holidays to allow their children to chase the dream. I'm assuming your son is an only child since I'm not sure I could justify robbing one child of their holiday to allow the other one to go flying! Quite a conundrum if you have more than one....
I hope he thinks the job is worth it when he finally gets there. Having been flying for 10 years now, for me it is definitely a JOB first - a means of paying the mortgage and feeding the kids. I suppose while I have huge admiration for your commitment there's something about your story which just rings a few alarm bells for me. I have a question mark over whether the reality of the job is really worth the level of sacrifice you describe. Particularly when you look at some of the increasingly depressing employment models and practices used by some airlines.
In any event I wish your son the very best for the future.