This is a tough decision for your son to make, if he does sign the ECP Scholarship then he will most likely not have the opportunity of AD.
There are other options, SMP can be an option and as long as he doesn't take a GRFD scholarship he would have a chance to compete for AD. Cadets sometime receive Battalion Scholarships once they have started ROTC, have good grades, APFT, and strong in ROTC. While this happens at many programs it's not a guarantee and he would have to work hard.
Your son should reach out to the ROO at the schools he listed and talk to them about the options before signing anything.