It is a hard industry to get into and the way it is going means that those who can throw the most money at it get ahead. By get ahead I mean get a job in the end, have met people who have spent 100,000 pounds + doing their ratings, type ratings etc and landed a job with someone like wizz air, this pays peanuts so will take a long time to, in fact an eternity to ever get ahead financially and will most likely produce more debt whilst working. The reason people do this is so they can gain valuable experience in larger jets and eventually move onto a better airline where they can start earning and paying back the money used for training. It is rare that once you get the basic FATPL that it is the end of the spending and the start of making a return, it is more often than not just the beginning of the nightmare that is usually the case with this job.
I am not sure if you have considered this but it is worth thinking about. If you are using equity in your home to fund this I assume that it will not be a case of just being able to keep throwing money at it till it works. I went this route and 10 years after starting my training am working for a major airline earning a descent amount, got a job 6 months after finishing, flying skydivers in a single engine piston aircraft and then kept moving up the ladder onto twins, turboprops, and now an A340. This route took me all over the world, north africa, scandanavia, central asia, asia, but I got there in the end and never had to spend anything on training after my initial licensing. It is hard but if as you say he has determination and drive he has a good chance of getting there in the end. If all he wants to do is fly an airliner then this is not the way to go but if he has wanted this since he was 5 then he should be willing to do it the hard way that does not require throwing more and more money after it.
I did my training in the US and it works out a lot cheaper.
Ballard Aviation, Inc. - Flight Training, Pilot Supplies and Aircraft Rentals
No I do not work for them but shows you how much he can get a multi CPL for. Conversion back to JAA can be found by searching this forum or online elsewhere. I never converted back to JAA as found a job flying N registered aircraft so the US license is ok. Just off the top of my head to convert back to JAA would cost somewhere in the region of 15,000 pounds but obviously look into it as that is just a guess. So if he could not find a job on N reg aircraft he is still able to end up with a JAA FATPL and you would have saved a lot.
Another option although the competition for this is tough is a cadet scheme. The only one I know of that is open to anyone and requires no financial outlay what so ever, even pays allowances during the training is the cathay pacific cadet scheme.
International Flights, Asia Airline Travel - Cathay Pacific International careers.
The biggest mistake people make is thinking that once you have a FATPL that is the end of the spending, it sometimes is but more often than not is just the start.
Edited to add:
My biggest regret was not going to Uni, received a flying scholarship from the RAF, applied straight out of school for a pilot position, did well in all the tests but they said I was still a bit immature after the interview for the job so go to Uni and come back after. I never went to Uni, if I had I think I would have had a good chance of getting into the RAF as a pilot. The RAF is not a stepping stone though, I really wanted to do that job so if your son has no interest in it it is not the way to go but maybe he should go to Uni. Mature a bit, join the university air squadron and do some flying, get a degree so he can get a descent job if the flying does not work. Most airlines let you work to 65, he is 18, there is no rush. If he does the flying now and in three years still has not found a job what will he do? Go to Uni when he is 21-22 after spending all of your savings? Go work in some low paying job as he has NO qualifications other than his FATPL but cant get a flying job? Have a back up plan, he is young, there is no rush. I was lucky but during the time from finishing my CPL to getting a flying job I was working in a factory, 12 hour night shifts, worst jobs in the world, I had nothing other than a CPL with minimum hours, a very scary place to be.