From everything I have read even if you have a full scholarship at university you are not guaranteed AD, you are given an oml and they pick the top 50%, and ECP are also put in that equation. So if you chose ECP your getting a 4 year scholarship so pick a Major that you can do on the civilian side as well, my son's major will be Pharmacist it can be used on both sides. I also read that once he gets to the University he is a commissioned officer so he does not have to do ROTC, he can put 100% into his studies.
It is true, cadets on the traditional scholarship at a 4 year university ROTC program are not guaranteed AD. At the end of their 3rd year and after they complete CLC they are placed on a National OML based on several factors. Many cadets choose to serve in either the National Guard or Reserves each year, the remaining cadets are placed on the OML to be selected for AD or sent to NG/Reserves. Your percentage is a bit off, of the cadets that actually compete for AD the number selected has been around 75% plus, I think it was even a bit higher this year.
The issue is the ECP cadets, while they tell them they are on the same OML, there is more to it. ECP cadets need to request to be released from their Guard obligation. QA1517 can give you more detail on this but here is the short version. Their son's ECP class had over 20 cadets request the release, the PMS only recommended 3 cadets, of those I don't believe any were granted the release. The next years class were all recommended by the PMS for release, again none were granted. All these cadets are now finishing up their degrees and drilling as a 2LT while they complete their degrees, when they graduate they will serve their obligation to the Army in the Guard/Reserves. These cadets were also told the same thing they are telling you, while everything they say is technically true, it hasn't turned out that way. In the end they can do what they want with these cadets since they are required to apply for a release.
I can only advise that your son does a lot of research and realize that in the ECP program things can change with little to no notice, and it's not always as they sell it. If your son wants a chance at AD Amy the best route is still to go through traditional ROTC or SMC
You are correct that once an ECP cadet leaves the Junior Military College they are commissioned a 2LT, they will however not attend BOLC. While they won't have to participate in ROTC they will have to attend weekend drill once a month and have summer training. The one weekend a month can take away some of that 100% focus on studies depending on what and where he drills. He will also need to find a Guard/Reserve unit that has an open slot for an ECP 2LT that has not completed BOLC, that is not always an easy thing to do.
Traditional ROTC is pretty straight forward, the ECP program can have a lot of twists and turns along the way. Again, don't take everything they tell you at these programs at face value, look hard behind the curtain.