At his age it's time to sit down and think about we're your son is now and does he really have a future in lacrosse! Does he have the speed and skill to play at the college level, and does he even really want to still play or he's making you happy. If he's on a B team and he's not one of the top two or three players on the team that might be a good sign he's had enough!
Fair points, there is 100+ D3 schools and as you say, does he want it. If yes, then he can play on the next level.