I wanna jump in here because I am an adult adoptee, and have contact with hundreds, if not thousands of other adoptees (mostly from Korea and Asia, as our experiences are connected).
It's okay for him to be "a little behind". Maybe it's because of early life issues, maybe it's just because every body and mind are different. The methods that schools use to measure things are really flawed and completely biased. There are so many ways of having and expressing intelligence.
That said, there will be issues. It's a really hard journey. I was a child who hid my pain and internalized all of the difference I felt. Certainly all of this was magnified by being a transracial adoptee. But from outward appearances I was a happy kid. I was also an anorexic teenager, suicidal through middle and high school, spent 7 years in an abusive relationship because I thought I was unlovable. I know others with experiences that range - but there is universally a struggle to know and love ourselves. It's a pain that haunts you - I don't know if I can even pinpoint what it is exactly. Some call it the "primal wound". There is also a lot to attachment theory that applies - when you don't get the love, or the love is mixed with pain or fear, or there is no stability, etc. But in this case, I think language and theory all fail. It's just traumatic. Abandonment is haunting. And no matter how young you are when it happens (I was an infant)...it can be a life-long issue.
I guess my point is...tread carefully. Don't force him to fit some ideal of "normal". "Normal" no longer applies. While working on his speech, encourage other forms of expression. Nurture his whole self, his whole spirit. Don't ever let him feel like he is failing your or letting you down or like you wish he would be like other kids. Don't let him feel like he's different because his mother abandoned him, but also acknowledge that he IS different because his mom abandoned him (does that make sense?). Let him have whatever feelings he has. Know that there is a lot he will hold back. Give him safe ways to express and explore those feelings. Connect him with other adoptees - particularly adult adoptees, not just things run by adoptive parents. Often, adoptees, we feel like we have to be perfect, like we have to over perform, over achieve, show no flaws - because we are afraid of being abandoned again. But at the same time, everyone tells us that we are no different. So all of those unpleasant, hard feelings we are having feel like personal flaws. I grew up with a lot of "well my mom tells me that i'm no different but i feel this way so it must be something wrong with me". Meeting other adoptees and having those feelings validated was life changing, if not life saving.
Anyway, I hope this isn't uncalled for. I just wanted to jump in and share from my (very different but also connected) experience.