With a PFD(and wetsuit in winter), on a well trafficed waterway. Also with experience on the water for 10 years.
There’s a difference between a swimmer and one acquainted with water. He’s been a boater,kayaked, fisherman, etc for 10 years and even is a first mate on a boat.
No, I knew a kid who was a fantastic swimmer, he went to Junior Olympics who decided to go riverrafting with his other friend. They both were found dead. Its not a good idea.
If they can swim, sure, why not?
If it’s a bad river then no. I would at least check on him every now and then. He should be a great swimmer and know what to do if caught in a current or falls into the water.
But since he’s been a swimmer for 10 years I assume he’d be alright.
EDIT: Yes, I know. He sounds capable of taking care of himself so I’d say yes. Just check every 45 min. or so.
As long as your son can swim an has some sort of contact between the two of you eg.phone (in a water-proof jacket he should be fine. you should go out yourself on the river first with him to get to know the water and area around it like landmarks places he can identify if he gets into trouble.