I know all the basics, have a routine, low simulation before bedtime, appropriate diet etc
One thing that stood out for me is you have a good bedtime routine for him which is great, keep going with that. Maybe add in some lavender based bubble bath and baby lotion, the scent is soothing and physical touch rubbing in the lotion is good for building attachment and is also very soothing.
It strikes me though that if his system is off kilter it may be that looking at it as a whole day issue might help. So lots of physical activity, outdoors where you can, combined with activities that are designed to help soothe him and help him learn how to relax. Children who experience trauma are often hyperaroused, it’s part of the physiological response to trauma and very natural, it means our kids are often hyper vigilant and need to learn how to self soothe and relax. So time outside in fresh air, gentle games or crafts, time cuddled up on the sofa with tv. Messy play is also good because it’s sensory and helps build the connection between his body and his brain.
I’d also keep the day as predictable and routine as you can, because that will lower his general stress levels, chat to him through the day eg now we’re going to X, then it’s time for Y. Keep things familiar, eg don’t worry about what he’s eating, or wearing etc - keeping things calm and stress free where you can will help. If you think of him starting with a much higher base level of stress that most children, he then has less capacity for any additional stress so it spills over into his sleep because his body is flooded with stress hormones which basically propel him into fight or flight (hence he can’t sleep).
There’s a particular type of therapy that might help called Theraplay, if you google you’ll find simple exercises you can do with him which are designed to build attachment and help sooth overwhelmed systems. You can also ask for Theraplay through post adoption support if you’re in England or through your placing agency elsewhere.
Avoid traditional sleep training stuff, those approaches will raise his stress levels and make things worse.
That’s much longer than I intended but some of it might help. Hang on in there.