Hey if Tails is good at hacking/computers you could get him to help out with your investigation. He could probably whip up a bunch of nice spear phishing/whaling emails if you have contact info for Shadow’s biological father. (Make a link that goes to a fake email login page, get his info from that, do some digital digging, etc.)
Or! If you’d rather try a physical investigation, see if Sonic can get hired by him and do some digging that way. He wants Shadow to be financially dependent on him so why not take advantage of that as a way to get information? If Sonic asks him for a job he may think employing your friend/roommate who helps cover expenses is a good way to make your more dependent on his money. It might put you guys into a better position to investigate him while also making him feel like he’s getting what he wants enough to let his guard down.
Yeah! He's a total wizz with computer stuff! He hacked our school computers and caused a whole school shutdown!
We are not having your brother set up a phishing scam for information on my father.
Well then let me work for him! It totally makes sense—I can bring in some money for us to pay for the apartment, your old man will think he's getting what he wants, and I can investigate!
But with bullshit reasons not to! Shadow, I'm good at this shit, I told you, I can get info out of people!
*Sonic blinks* ... But you said you weren't worried about him doing anything to me?
I'm not! Right now, you're nothing to me, but if you're financially supporting—
*Shadow pauses* ... In his eyes.
I didn't mean... that we're not friends, or... well, to him, we just happen to be roommates. It's not—
No, no, I get it. All good dude.
I'm gonna look for a job that gets me close to him. I'm an adult, I can take care of myself, and I can make my own choices.
It'll be fine. Seriously.