Chico, with your son's likely letter, he's in. You have nothing to worry about as long as he doesn't get arrested, kicked out of school, gets a bunch of D's on his report card 1st semester, or posts something obviously inappropriate on social media.
The timing of likely letter after ED deadline may just be related to when he submitted his complete application. The Ivy's issue their likely letters only after the full application has been submitted, if everything is received by admissions early, then likely letter can be issued anytime after October 1.
In any case, the likely letter is an offer of admission contingent on the things mentioned. I think the main reason schools only call it "likely" is because they want to say they have one official date for notification of all students, but the purpose of the likely letter for athletes is to let them know they are accepted so they can let other coaches know their recruiting is done.