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First thing I would do is frankly assess why your son is still playing a year down, where he stands in relation to those players, and how those players stack up as college prospects. Second thing I would do is talk to your coach, tell him your son's goal (if it really is his goal), and ask his advice on how to invest your son's effort in that crucial summer after his junior year.
Once you've done those two things, you can look for the following indicators that it's time to move on:
1. If the team won't have the reputation or competitive record to attract college coaches to its games at major tournaments. It's very unusual--possible, but unusual--for a team made up of guys who have played together since little league to still be together in high school and be an effective vehicle for college recruiting. Is this group really capable of going to East Cobb and doing some damage? If not, get on a team that is. With 200 teams in these big tournaments, college coaches need a reason to scout your games. Usually that reason is if a team has the best players from a certain broad area.
2. If the coach doesn't have a plan or the personal commitment for getting your son the right exposure. My experience is that the right travel coach who really is personally committed to finding the right fit for his entire team--and has the network to do it--is worth the price of playing on a high level travel team. Does your coach know the current recruiting environment and have the contacts to get college coaches to come to his games?
3. If your son is more advanced than his current competition. If playing down a year group has him dominating younger opponents, he needs to swim in a deeper pond. A good rule of thumb is to play on the best team he can get adequate playing time.
Hope this helps.
Best wishes.