As you must know, D1 coaches can’t communicate with recruits other than to invite them to ID camps until June 15 after your son’s sophomore year. I agree with @gkunion that if there is something personal in the email (“We saw you at XYZ tournament and were impressed with your play in the midfield, especially your willingness to tackle”) then that means your son is definitely on their radar. However if it is just a nonpersonalized invitation to an ID camp then it usually means nothing.
Because D3 coaches don’t have that constraint, they can communicate with players any time. Is your son getting specific feedback from D3 programs? If yes, then obviously he’s on their radar, and also may be getting some attention from D1 programs. If no recruiting interest from D3 at this point then probably none from D1.
I would say the single best thing to do at this point is to get your coach’s assessment of where your son falls on the recruiting spectrum and go from there. It doesn’t hurt to email coaches before tournaments etc, you never know when one will “hit”.
There are all kinds of strategies and goals with athletic recruiting, you have to figure out what makes the most sense for your son. My son’s goal is to get into the best academic school he can, using soccer recruiting to boost his chances. As a result, although he’s a low D1 player, and has received some interest from low D1 programs, he’s not pursuing those schools, he is targeting high academic D3’s where he wouldn’t get in without soccer, and where his soccer level makes him an impact player. Other kids on his team want to play at the highest level possible and are less focused on the academic side.
ID camps where you get specific feedback from the coaches is a good idea too. My son attended 4 ID camps between his sophomore and junior year, and we tried to do a mix of small/big urban/rural so he could check out the different colleges as well as the soccer. We added on an extra day at each school so he could do the tour and attend an info session.