The next spring we went to watch my nephew play in a D3 game. It was ok baseball, but some of the kids wouldn't have played on our HS team.
Don't be misled by this statement. There are many D3 baseball programs where all the players were all conference in high school. There are D3 programs where some of the players could have played D1 but chose the D3 for its high academic standards and reputation.
That said, when my son was in high school we stopped while biking by a local D3 game. After two innings my son wanted to leave. He felt neither team was good enough beat his high school team. The reality is there is probably a place to play for any competent high school baseball player who wants to play college baseball.
Tell the high school and summer ball coach he would like to play college ball. Get their reaction. Don't take it as gospel. But it's a start. Send him to a local showcase. Find out who some of the players are at the showcase. Compare how your son matches up with them. Do you have a good friend who knows the game who can be honest and you can accept it? What position is he projected to play in high school? Most D1 position player recruits (outside catcher) are up the middle players (shortstops, centerfielders) or they're power hitters.
Chances are you will get a better idea of his ability by the end of next summer. I was amazed how much bigger and stronger my son and his friends got by the end of soph year. While some kids join showcase oriented teams the summer after soph year it's not absolutely necessary until the summer after junior year.
If there are major showcases near where you live go watch one for a few hours. Try to figure out how your son stacks up. Talk to parents. Talk to coaches. If they're busy don't be offended if they blow you off. When my son was in high school I attended D1 and top D3 college games. The most informative question I asked was "How did your son end up here?" And if they don't include it, "What showcases did he attended?" It's an open invitation to a parent to talk about their kid.