Is there a reason your nephew can’t sleep in house?
We do allow him to sleep in our rec room/guest room. He and his family have visited us before and know we have a spare room. Undoubtedly that’s why he drove 1000+ miles to come stay with us without our agreement.
That was presumtuous as hell. They have the right to not allow a person in their house. I have had personal experience with letting people into my house out of the kindness of my heart, then being totally raked over. Either there is thievery, a sense of entitlement, or the guest that never leaves -- making you feel like an outsider in your own home -- and I totally understand letting the guy sleep in his truck.
You know, sometimes if you do that, they just never leave. And in some states, once you let them hang their hat, they have all the rights of a renter and you can't legally boot them out. So learn a little before you climb on your high horse.
Sometimes it's better for the young man that he doesn't. There's no serious harm that comes from sleeping in your vehicle, but it does serve as a constant reminder that it's expected that he shape up and get a job - you know, start being responsible for himself and all.