2020s in winter 2018-19

It's not a stupid question and one we always wondered about. To answer your question I would sit tight for now. No need to update if there's nothing to update on. A text message saying lifting/training is going well will most likely go ignored. If you want you can probably send a text now saying it was good talking to you at the showcase last weekend - did you see anything I should work on in the offseason? Maybe not. We did this in person and the question was always well received. Others can weigh in here. 

The good news is that the northeastern schools aren't really recruiting in the winter months. So your son will not be losing his spot or interest by not updating them. Maybe a text when HS season is starting back up saying my I put on 5 lbs of muscle or lost 15 lbs or gained 3mph, we open on XX/XX look forward to hearing from you. 

I hear some parents say they texted the coaches every week updating them. Whether it was high school games, summer games, winter training, etc. The coaches don't care. They're not looking for friends, they're looking for players. They don't want to hear about your hs game or how hard your teammates battled. If you are texting with a school for months at a time and they have not made you in offer yet, they're waiting for others to say no first. 

My oldest is a year ahead of your's. Slightly different scenario, but enough similarities that it might be useful:

• Junior year fall he played scout ball and travel ball as a position player only -- he didn't pitch from mid-July to mid-November -- so that limited pitching interaction with coaches (obviously)
• Once he started pitching again (Nov.) he sent email updates maybe once per month or every six weeks, with video highlights -- for your son, maybe that is Jan or Feb? Not sure when your son will start pitching again or when your HS season starts
• Over the course of the winter, I know there were communications going on between my son's HS coaches and various colleges, too, so if your son's HS coach can help, that's a big plus
• Once his season started (Feb) he sent more frequent updates -- weekly texts to the coaches he was texting with, emails to others he was trying to get attention of. 
• Don't assume that they won't see him until the WWBA.  Your son should definitely send his HS schedule and when he is pitching, If they are interested, they'll come and see him -- well, some of them will come see him (obviously, the closer the school, the easier it is to do that). First week of March, as soon as the quiet period ended, an Ivy RC flew out to see my son pitch in a HS game. Since you are in the South, it's not super close, but they can and are getting from the Northeast to the South to see kids.
• If your HS (or its opponents) webcasts any of your son's games, that is another opportunity to "get in front of" college coaches. For my son, it was a big plus in getting seen by colleges on the other side of the country -- he would send the link and they could watch an entire game if they were so inclined
• If they have invited him for UVs, you may want to consider doing those over the winter. If a kid from the South is going to go to school in the Northeast, it's worth seeing the school in the winter.