Our experience was that northern jucos mostly - but not all - pretty 'relaxed' in their recruiting. It seemed to me that they were waiting until after the early signing period for high school seniors so they wouldn't be wasting their limited recruiting resources. Then, with the early signing period over, they started pursuing the players they wanted from those that remained available. Some jucos are more aggessive in their recruiting and it shows, typically, in their win-loss records.
To answer your question - yes, my feeling is that they focus promarily on high school seniors. And, they begin their recruiting once the early (Nov) signing period is over.
Also, be aware that jucos are also looking for opportunistic roster additions from D1 players unhappy with playing time, etc. and want to switch. It was a big surprise for us to see how many D1 transfers came to jucos.
As a junior, I would have your son focus on his grades, and continuing to develop as a player. By the way, junior college academics can also be very rigorous, so be forewarned. And, mom and dad won't normally be there to emphasis good study habits.