don't mean to sound mean, but at 21 your son should be doing his own research and figure out what's going to work for him.. I echo other posters in regards doing the medical first and taking a trial lesson at your local club. For everything else, really it depends on the budget and whether the plan is to combine studies with work or not. If you're going to pay for his tuition and support him financially, just send him to one of the EU integrated schools that actually have ties with the airlines. At the same time remember that these programmes are tough and if your son is not the brightest tool in the shed, they will drop him mid point or not take him on board at all.
If the budget doesn't cover the leading integrated schools or he's going to be working at least part time, go the modular route, do his PPL at the closest club to home possible. Then with a PPL sorted, whilst doing hour building/ATPLs he will get to know people, see what the latest trends are, where people are going to do their CPLs - at that stage he should be able to make an informed decision himself.
Personally I wouldn't bother with US -yes, maybe it works out cheaper in some cases, but the hassle with the paperwork and getting everything converted would be too much for me.. others might think differently..
anyway, Pilot Careers Live event is coming to London this weekend, why not pay a visit and see what the story is -
https://www.pilotcareernews.com/live/