I echo much of what 3n2 said. You seem to have a pretty good awareness of what's up and you seem to have him in a good situation with a well connected travel org and he is getting attention, albeit no solid D1 offers.
The few red flags I see are tied to panic. He's a soph and you have already had a "revenge tour" and you are reacting to others on his travel org getting offers already. A soph in HS and you're already planning a redshirt year in college. Take it down a notch. I'm guessing part of the problem is that he started getting some attention very early on, before he started HS so you started feeling like you had to get him on the fast track recruiting timeline.
Your son is 15. Many don't come anywhere near hitting physical maturity until well after that age. He will mature physically at HIS own pace. I too think at least one key factor in not getting those final offers from D1's is the fact that he is just shy of targeted velo's. Most important is to keep his arm healthy and factor in the 40 yr plan as part of the recruiting/college search process.
He is a lefty that is close with his velo and otherwise proven as an effective P against strong competition. Even if it takes him another year or so to find those extra 3-4 MPH, he will most likely have very good options when he does. If you let the pressure of the moment push him to continuously chasing the "next big event", you risk pushing the arm beyond what it is ready to handle. And it will probably help a great deal if he knows from you that it's OK if things don't happen in the next 3 months, 6 months, a year. With continued efforts, patience and precautions, he can still do great things.