Pima is right. You have plenty of time on your side. However; if your son is the type that's not going to have any problem getting the CFA completed with excellent scores, there's nothing wrong with getting it done and out of the way. Like Pima said, simply don't submit the scores online until your portal is open for you to do so.
At the same time, most schools are open in the summer months for many reasons. If you coordinate with your school admin and guidance counselors now, I'm sure you can work out a time when you can go there in the early part of summer; after your portal opens; and have them ready to send your transcripts, class schedule, school profile, etc. And in the summer, there's usually at least 1 or 2 coaches and/or PE teachers available at the school. Our football coach was there doing strength conditioning for the football players; then 2-a-days later on; etc. Maybe you can arrange with them to do your CFA. Depending, most of them are allowed to give you the CFA test.
I applaud any candidate who wants to get the application done early. I highly recommend it. But in order to do so, you really have to be on the ball. When my son applied, the portal opened on June 15. He was 100% completed with his application before July 31st. Less than 45 days. This allowed him to enjoy the rest of the summer; use August and september to get all his other college applications completed and sent in; and to really enjoy his senior year of high school. Basically, by the time school started in September, he was completely done with "College Stuff". But you have to determined. You have to have everything; especially with the school admin; lined up ahead of time. You can't just expect to walk in during the summer and expect everything to fall into place. If you have the admin and counselors lined up; your teachers recommendations lined up; your CFA lined up; your writing sample and the application lined up; then it's very easy to have the entire application completed in 45-60 days. That includes getting scheduled for your medical physical. And; if you're on good terms with your ALO, you can even get the interview out of the way. And with my son having a Presidential nomination in hand, he was eligible for an early appointment. Which he did receive at the end of October.
So yes, you can definitely get the CFA and the entire application completed early. Much earlier than most. But you have to really be prepared for it. And that starts today. Not in May when school is getting ready to get out. When teachers and admin are frantic trying to get the senior class graduated. You need to speak with the appropriate people now, so you can agree on a followup time in the summer to get all this completed. It can be done.
But also realize a couple things. There's only 2 things that a candidate/applicant has 100% complete control over. The CFA and their ACT/SAT tests. Some say their GPA, but once those grades are in, you can't really change them. But the act/sat and CFA you can control. But you only get to submit the CFA 1 time. Whatever the scores are when you submit them, that's it. Some will debate that the academy will let you RETEST the CFA. The truth is; whether or not you get to retest the CFA is totally up to the academy. If they like your application, and you appear to be one of the tops that they want, and your CFA isn't good enough, they will allow/ask you to retake it. But this isn't 100% guaranteed. It's up to the academy. They've been more inclined the last couple years to allow more retesting. But I would not bet on it. If you have a very average application, and your CFA scores aren't high enough, the academy could possibly not offer you a retest. In which case, you could be deemed "DISQUALIFIED". End of story. So before submitting those scores, you want to make sure the scores are the very best that you can do. Not average, not the minimums, but the BEST YOU CAN DO. You can practice and take the cfa as many times as you want. But you can only submit the scores one time. Unless the academy offers you a retest. And I'm sorry, but I would not bet on that as a backup.
So, unless your son is a major athlete, who can get close to maximum scores on the cfa, you might encourage him to practice it until he can't do any better. Again; he can do all that now, and still arrange for getting the application done early. It all depends on how determined he is. Best of luck.