In our area, most (HS draftees) are doing same as your son. All of the 2014-2016 draftees he knows are back in town working with the same local trainers they did through HS. Most of them are trying to rest up from the grind their bodies took from starting their January HS season and playing to mid October in the final instructional, while putting on some strength and weight to get ready to grind again. A couple are rehabbing from injuries/surgeries from the season.
Many of the college draftees get apartments near their school and workout in their facilities.
Granted, throughout the sunbelt, the facilities and density of these guys tends to be a bit greater, so subsequently, the options tend to be better as well.
Not aware of any specific programs like you ask. Here many of the MLB'ers return to the area and are working with same trainers as our young pros and HS'ers. Many of them will get together to hit and throw as well on their own.