What to think?

Most coaches don't include cell number for every correspondence/response they send. Usually means son is on radar. Cell from HC or RC carries more weight than the third assistant, who may have represented school at showcase.

Had son previously met with any of these coaches that he's receiving these emails?

If they included "you can call my cell phone" in the letter, there's a VERY good chance it's legitimate interest.  If your son is interested in the school, have him call them RIGHT AWAY, even if it's an assistant and not the RC or HC.  Even assistants don't normally just throw out their cell phone unless there is real interest, and in most cases, they've probably had some discussion with either the HC or RC before they emailed your son.

Concur with above statements.  If cell phone included, it's likely more personal.

Point Blank: if your 2019 is hitting 87, he has upside, separately, its about command and projectability, at this point he's hitting the velo test to generate attention. 

How big is your son?  Been to any showcases like PG or PBR?  What showcases has he been to generate the emails?

Agree. Cell phone number means "call me." In about a week it won't matter, as they'll be able to call, text, and email your son (Sept. 1st).