Boy, is this a loaded question! but a good one as Justbaseball said. Whether it's a necessity or not depends on a lot of things- does your son know where he wants to go? how long is his list? Does he want to stay local or does he want to expand his geography? How connected is his HS coach and does he promote his players? I can tell you in our case, travel ball helped tremendously. We rarely see a college coach at our regular season games, we are a 'northern' area, and our HS season is only 18-20 games depending on weather. Our HS coach doesn't view promotion of players as his responsibility- which is fine I guess, it's not what he's paid to do. So in our area, and for our situation, I'd say yes, travel ball at some level is a necessity. Others with prominent HS baseball programs, in "baseball" areas probably would differ in opinion.
Just my two cents-