We've been on an ACA plan for over a decade. No navigator or assistant needed. If you miss the 12/15/25 deadline, he can sign up by 1/15/25 for a 2/1/25 start. Not ideal, but better than waiting a year..
Yes, most plans have a limited network, some worse than others. For life threatening injuries/illness, though, they will stabilize and transfer. Yes, there are horror stories, and there's also help for those caught up in those, and charity care is also an option, but that's just something to stick in the back of your mind.
Have you gone to healthcare.gov, put in his zip, age, and income? That's where you need to start.