I was a 'natural' at many sports, and an actual olympic hopeful in 2.
What you're looking at is called Burnout.
It's not fun.
GREAT coaches deal with burnout all the time with their stars and they *back off*. Kid takes a season off and still plays/swims/etc but NOT competitively, and typically in a different venue. Swimmers usually end up doing ocean sports (surfing, sailing, scuba), or whitewater, or diving. Something that keeps them in the water, but is new and challenging and FUN. It takes about 6mo to a year to "get back" to where they were when they took a break and got the FUN back, but it's 100% worth it.
Although, many make the switch permanently if they've been suffering through burnout for some time. GREAT coaches spot burnout early (and are trained to do so) so they can get their player back, and often incorporate the "blow off steam" activity into their training. Unfortunately great coaches are few and far between. And because the kids have LOVED this sport for so long, and because they don't want to disappoint their parents/coaches/friends, they usually wait until they're about to completely melt down and lose it before they speak up.
Having been there in one sport, it's a devastating thing to lose your love. Your son is probably hurting bad. Broken hearted, but far, far too burned out to even be able to think clearly about it. There's also the guilt. Comp sports are expensive, and we KNOW our parents have invested their time, money, and energy into us.
It may not make sense to nonplayers (I'm hoping/bet it will make sense to you), but this process is like a loved one dying, or an affair in a marriage. It HURTS. Your son is hurting. You can fix that. That's how you don't get angry about it. It's not about committing, it's not about excuses...it's about learning balance and learning how to keep your own sanity.