there are scads more trails at Yosemite that at Isle Royale
Yes, Yosemite is a hiker's paradise and this young gentleman would have the chance to hike straight out of Tuoloume meadows for as many miles as he cares to go, as deeply into the (truly fabulous) Sierras as he wishes, for as many days as he can carry food to cover - two weeks worth if he's willing. All this, and public transport from the airport to the trail and a chance to resupply without leaving the park, then go off in a different direction for another two weeks!
But I find it is mighty hard to deflect a person who has fixed an idea in their head for a while and started to build a dream around it. We may believe this would be a fantastic choice for him, based on what he's said he wants, but if he is going to accept our ideas on this, he'll have to trust that we know a lot about this matter that he doesn't yet have the knowledge to conclude on his own. That's one heck of a lot of trust for someone who is specifically trying to demonstrate his independence.