D3 teams seem content to let players mature in junior, and then snap up quality players who couldn't quite land a D1 spot. But it's possible to play D3 before ageing out if you son is good enough ... meaning he is one of the better players in a good league.
The reason you don't see many D3 commitment announcements now is that players are still pursuing their D1 dream. You start to see the D3 announcements in the Spring and Summer.
I would have him start reaching out ASAP. You never know.
Unfortunately, the desire to major in business/finance eliminates several D1 and D3 schools. So many of the liberal artsy schools - even big ones, and several of the super-exclusive schools - lack business/management/finance/accounting majors and only offer "Economics." While economics can be a solid, business-focused major, it isn't always. Some schools treat it more like a social science. Babson might be a good option for you because it has a D3 team and a business curriculum.
My advice is to pick the college first, apply and get in. If they have a D3 program, great. Go for it. If not, ACHA hockey is a great option. Kids still get to practice three days a week, play on weekends, travel and play descent hockey for their school, but they are not slaves to the team and can enjoy more of what college has to offer.
My son was approached around this time last season by a private D3 school to play this year. So it does happen.
However he decided he wanted to play another season of (tuition free) juniors instead. The good is that he is having a great year, the bad is that the school filled that roster spot. So if he gets an offer he wants, take it. Good luck.