DS is in a different financial position (blended family) so in the event of no ROTC HSSP, the enlistment and application to AFROTC POC-ERP utilizing TA/GI Benefits may work best for his situation. I'm not familiar with the TA or GI Benefit processes and amounts, so need to do additional research to help identify options for that course.
GI Bill is available after 3 years of AD service (Guard/Reserve get a reduced amount, depending on number of days served after 36 months).
ADAF tuition assistance (TA) pays 100% tuition and fees up to an annual cap, which typically increases a little each year. It's available after completion of technical training, usually around 12 months in. GI Bill
can be used while AD, but it is generally discouraged due to the availability of TA.
Every AF base has 3-4 schools that offer on-base college courses tailor-made to fit with TA tuition constraints. As long as your son pursues a "normal" major, he shouldn't have issues completing enough of it to apply for POC-ERP or
ASCP/SOAR ( two other similar prior-enlisted AFROTC routes).
Ultimately, he may find it faster to just finish his degree on AD and apply to OTS (takes 2 months to produce the same result that takes 4 years via AFROTC and USAFA).
The biggest challenge to AD college attendance is the ops tempo. If your son gets an exciting (or high-deployment) AFSC, it may be difficult to prioritize sticking to the college regimen. That was certainly the case for me for the first couple of years.
One final route would be to look at enlisting in a Guard unit. Many states give free instate tuition to ANG and ARNG members at all state colleges. There is a mechanism for Guardsmen to maintain their Guard status while also taking the prerequisite ROTC classes, ultimately commissioning into either the ANG or Active Duty. This is called "purusing status".
It would be the most flexible option for someone who can find a position they like in a Guard unit for a state that has such a program as there would be
no need for a scholarship and
no degree restrictions. The drawback is that it can take up to a year to get a Basic Training date in the Guard, and no benefits are typically awarded (including pay) until BMT completion.
Tons of options out there-- don't get tunnel visioned into thinking AFROTC and/or HSSP are the only path forward. Keep asking questions as you have them!