The hitting behavior is common is this age group. Needs to have limits set.
As for co-sleeping, one of the problems with co-sleeping (whether bio or adopted!) is that they search for the parent all night long. Even if he was bio, he may know you will leave so he wants you to stay with him from the beginning. Also, common that he is searching for you at night. Wants to sleep on top of you so he knows you are there. If a child is used to sleeping alone, they don't have the "searching" behavior. Ever heard parents who co-sleep talk about how one is slapped and one is kicked all night? Very common! So the sleep behavior could be that due to his transition, he is searching for you and needing comfort. But it could also be a result of co-sleeping.
As for attachment, I'll share some info that helped us. When Grace came home, she didn't sleep. Was it attachment or a medical problem? IAP pointed out how Grace showed all good signs of attachment during the day. At night, she woke and the only thing that soothed her was a bottle. I could walk into the room, but the bottle in her mouth (never pick her up), she would take 10 sips and go back to sleep. The IAP felt that since during the day she sought me for comfort and was comforted by me, she would do the same at night. At night, all she wanted was the bottle which was consistent with pain from reflux. She also had many other signs of reflux pain and a milk scan (radiology study) to confirm the reflux.
So the big question is, What is his behavior like the rest of the day? Do you think he has good attachment or possible problems outside the hitting, spitting out and sleep problems. With Grace, I don't think the bonding could have been better. I had felt an immediate bond with her. she had great eye contact, sought me for comfort and appeared to be comforted by me. Of course, I worried about the sleep being related to attachment, but the IAP helped me see it was related to the reflux. When I went to Korea 4yrs after we adopted Grace, I met with her fm again. In my conversation with the fm, she described Grace's behavior to a tee!!! I then knew beyond a shadow of a doubt, that Grace's sleep problems had been happpening in Korea (due to reflux) and nothing to do with attachment.
You may get different input from each of us. But I think all of us would agree to go with your gut. If you think it's more than age appropriate behavior, I'd seek help from an IAP or AT.
DS, 11yo, Bio
DD, 7yo, Korean adoptee