Frankly Mr Shankly has given an excellent description of what modular training entails. That’s pretty much how I trained, for a total outlay of around £45k, including accommodation for time spent away from home. I qualified with pretty much minimum hours, and the only avoidable cost was £1,000 spent on retesting part of the CPL exam (which is very easily done and should be budgeted for), so £40-something thousand is probably about as cheap as it can be done. Note that was around 2 years ago, so the prices are reasonably current - although the job market has changed dramatically since then. I also undertook the aptitude testing mentioned by JohnRayner and would strongly recommend it.
The best advice I could give, considering the woeful state of the economy and your understandable desire to avoid writing cheques for £80k, is for your son to take his time with training, and avoid the lure of the big schools’ glossy brochures. When I qualified, the UK’s four biggest employers of pilots (BA, Ryanair, Easyjet & Flybe) were all recruiting in significant numbers, but even then it was tough finding a first job, and many people struggled. Today, recruitment is pretty much at a standstill, so the task is nigh-on impossible. Things will improve eventually, and if your son is currently doing his PPL and spreads his commercial training over a couple of years, his timing could be good. Beware, however, qualifying when the job market is stagnant. Airlines value currency ( i.e. recent flying experience) very highly, and it is difficult and expensive to maintain this at your own cost while you wait for things to improve. Someone who qualified last week will be more attractive to a recruiter than a candidate with a year-old licence and 20 hours’ flying experience since then.
Best of luck, and remember never to trust any ‘careers advice’ from a flying school (BGS excepted – Alex is a very straight guy!)