I've an adopted nephew, actually my sil's kid but my brother adopted him as his son when the kid was about 2 or 3. The parents made what I thought was a mistake of not telling him. That only became awkward for me in discussions about looks and especially I was uncomfortable when mom wound up with Alzheimer's because I could see how worried the kid was, watching his grandma deteriorate, and I'd have loved for him to know it wasn't in his genes so he wouldn't worry about himself. But I played it how his parents wanted.
Then he developed into quite a bit of trouble, possibly even some psychopathy involved which we hope he eventually learns to control. Anyway, the kid's off at school across the state by himself when his half sister--his biodad's kid--from across the country, who was told to never tell him of their relation until the parents think him ready, finds his facebook page and lets the cat out of the bag. That night the kid winds up with a group of bad guys, breaks into a car and gets thrown into jail, off campus and out of school. So that news didn't go over well at all.
He felt betrayed, lied to, alone, all that, even though none of us ever treated him any differently than his 1/2 siblings who are blood. Could be we screwed up by not telling him early. Hard to judge. He was always a bit of a screwy little kid. Maybe things would have gone better but maybe worse, we don't know.
I don't think he's felt like family since. I tried calling a couple of times but he doesn't return my calls. When he was having trouble getting along with his family I invited him to stay with me in my house or in a cottage I've got and my brother would have sent him to school here but he refused that too. Hopefully he'll come around some day. Because he didn't miss out on any love in life. He was adopted into a very loving family. And material wise, my brother gave him everything, his own apartment, education, a nice car, all paid for. Sure he didn't get the pony my niece got, but he didn't ride. He doesn't even like horses. It's all in his hands now. We hope he comes around, but it is scary.
I've met other guys during life though who embraced their adopted families so this case might just be off. So I can just share that and advise talking to his family and if they advise further, to the kid. You don't say how old he is. Maybe he'd be okay with it or maybe waiting might be better.