The Stanford camp is not only about being seen and getting on the radar, but it is a learning experience about what college baseball is all about, living in a dorm, etc. For the value I would recommend Stanford over Headfirst. You can do a search here and see all kinds of information on both of them.
I would also look into the Academic Game at the Arizona Fall Classic. He may not make the team but just trying out and getting on their list will get him exposure.
You are actually in a great position since you are focused on D3 schools, so I would have him (together with you) make a list of schools he is interested in and have him contact them directly. Send them a short profile. LHP will always get coaches attention. You should also discuss what major your son might be interested in since this will direct him to particular schools as some may not have the major your son is interested in.
Your son would likely be a target of the service academies, but this is something he needs to think about and discuss. Decisions for these schools go well beyond baseball, so if he is interested in one he could also contact the recruiting coach.
The concentration of D3 schools in the West are SoCal, the NWest, spread out in Texas, the South and then the majority are in the NEast.
I will go dig up a map that shows all of the schools and post it here to give you a visual.