What's the drive to have him talk about his birth parents? He may really not be ready to go there just yet or may have no idea where his anger is coming from so making links back to birth family may not be helpful for him. Apart from anything else his anger may have no relationship to his birth family and may be more about the daily stresses of school etc.
How's his emotional literacy? I know my 6 year old can really struggle to link feelings in her body to how she feels emotionally and to find words for emotions (e.g. doesn't link sore tummy to feeling worried about going to the dentist). She literally had no feeling words when she arrived with us so we've spent lots of time working on it - wondering aloud if she might be feeling X about Y, being explicit about our own feelings and really helping her to sit with herself. So lots of comfort and holding when she's angry, zero tolerance of hitting of any kind, use of time in with one of us - not sitting with her but her sitting with one of us in the room. She found that really hard for a while but did use the space to calm down and think about what had happened. A lot of her violence was directed to her little brother but now she can manage herself and actively works to not hit him (e.g. leaving the room, jumping up and down etc).
She and her brother are nowhere near being able to do life story work - I anticipate that process won't happen for them for a while tbh and I will very much be led by them. At the moment we're open about adoption, we build a family identity (lots of family hugs, teamwork etc) and we answer any questions the kids have to the best of our ability. Anything more than that would, at this stage threaten their sense of permanence with us so we'll wait a while.
You might find he's also going through a stage of cognitive develop where his brain starts to understand his world differently so his understanding of home and family might be changing and I'm thinking that could be very scary for a little one who's world hasn't always been safe. Can you reassure him of his place in your family and help him identify ways that he belongs and where? Are the school supportive of his particular challenges e.g. Are they making you aware of issues because you need to know or because they're frustrated? My DD school tend to be quite creative in how they schedule her work time, support etc to account for the way her mood and energy levels change across the day and the week. She works hard to hold it all together in school and literally explodes at the end of the school day - sometimes she really needs to pick a fight with me just to let off steam, which is fine - I'll let her pick, be cross with me within clear boundaries (e.g. no hitting, no swearing, no throwing things but shouting is ok but mummy might be cross back) - once that's out of her system, she has a bit of a cry and is back to normal. Not the easiest transition home but it is getting better and we're finding other ways for her to decompress as time goes on.
It's hard when their need to express strong, often unknown emotion presses our buttons - get support for yourself, I've started seeing a therapist for me, not my kids. Its such valuable space to catch up with myself and to express my own strong feelings safely.