"Adoption" - Wouldn't work, because he'd need to fill out extra paperwork for that. Go through extra processes that'd register them both.
"Guardianship" - Literally the exact same problems as above.
I mean come on, was reading the freaking text presented to you in the chapter that fucking hard? Or did the brains of most of the people in here, get blindsided by the word 'slavery' and go full-tilt without even giving it some thought?
Swear to god, some of you are so drunk on the feeling of "how right you are" whilst being unable to simply comprehend and piece together a few simple sentences.
They didn't have identification documents, and they needed that to freaking. You can even see dude saying he left his GUILD CARD back in the forest with his old bag. You know, to make it clear he was fucking dead?
I don't know about how many of you have dealt with legal processes, but paperwork gets backed up and kept around, often a copy for you, and for the person you're signing it for. Meaning that, if those copies get around, he'd have to deal with an even bigger pain in the ass than he already has. The Special Slave thing allows him to not have to deal with paperwork, which means it's extremely unlikely that it'll be reported back to the king.
Also, a little tid-bit of knowledge, slavery is fucking ancient, and has apparently been documented to have been in practice since - 3500BC. Some of the most prominent slave traders dating back to a couple of hundred years ago, weren't even the fucking Europeans/early American settlers that y'all are losing your shit over. It was the Arabs. Even very recently, about only 100 to 150 (not exactly sure cuz I couldn't care less) years ago, the Arabs were still trading slaves. Talk shit when you actually know shit.
Another thing? If you're gonna get up in arms about something simple like this, then I hope y'all will be prepared when you read an isekai with truly, truly different moral values from our own world.
Sorry but I'm tired of seeing a bunch of y'all acting like children, not even having any decently valid criticisms for the most part. That one guy who was talking about raging teenage hormones, tho? He had a point with that statement. Try and be more like that.
Peace.