The first reason is simply economic: English and Spanish clubs have enormously larger budgets and can offer salaries that Serie A hardly ever reaches, even for young prospects. Add to that the Premier League especially invests heavily in foreign talent because it knows it attracts more global visibility, and you understand why a 22-year-old guy with a good offer leaves. For your son though it doesn't change much following different leagues, what matters is that he watches a lot of football and understands his role, but if he really wants to pursue a serious football career, unfortunately knowing English (or Spanish) becomes almost essential these days because the best opportunities are there.