I say it depends. After he gets a couple of years older what are the odds that he will 'out grow' the one rifle?

As for converting your other one you would need a bolt and mags. I wouldn't see the need to get a high dollar barrel for the intended use...

On balance for the use case you mentioned I would definitely lean towards the bolt action. I would say 80/20 in favor of the bolt gun.

Add a suppressor and you will be right there.

Making a 16"gasser in 6 ARC run might be a little more involved.