There are some good parameters here to give you a general range of what gets a kid in the discussion of being admissible to HA schools. As others indicated, admission rates vary widely even among schools that are considered HA, but test scores and GPA are the criteria that the coaches will consider as the first factors in deciding if a kid should be recruited at a HA school. One HA D3 coach told us he went through a showcase roster that listed the kids academic scores they reported on their application and then crossed off all kids he knew would be excluded from admission so he wouldn't waste time watching them.
Rest assured, if a coach is sincerely interested in your son at a HA school, he will ask your son to send him a copy of his transcripts, test scores and Senior year schedule. I highly recommend those attending HeadFirst or any other academic showcase to bring envelopes with those docs sealed in it ready to hand a coach if he shows interest and starts asking questions about academics. Almost all of my 2017's interested coaches would run those documents through Admissions for "pre-reads" and then those that got through followed-up and those that didn't, either he never heard from them again or he got a "no thanks" reply. I was surprised he got through some tougher schools and got rejected by others I thought wouldn't be as tough, so you never know.
Finally, if your son is on the margin with the test scores, have him retake it and see if he can get an improvement. Going up even 2 points on the ACT can make a huge difference, especially in terms of any academic money he can get.