It is extremely normal for adopted kids to need to cycle thru their grief and their issues over and over again. There may be a lull of 6 months or a year where they don't ask any questions or mention their birth family.

Then they may think about it lots for several months, and ask lots of questions.   Whether or not your child sees it as an issue, it IS a part of his life.   I think the more open and accepting we as adoptive parents are, the more likely they are to always bring their feelings and concerns to us. 

They say that even kids who do not mention birthmom ARE thinking of her at times, so from time to time (a few times a year), even if my kids don't talk about it, I will bring up birthmom.   On their birthday I will say something like, "I bet your birthmom is thinking about you today." 

Or when they show some skill, I will say,"I wonder if your birthmom was good at that too."  The other day when I was lotioning my 3 year old, I said,  "Did you know that your birthmom has pretty brown skin like you?" 

I don't dwell on it endlessly, and if they do not add to the conversation, I let it drop.   But I want them to know that birthfamily is an okay topic to bring up. I think that if I talk about it from time to time, it also normalizes it for me.   

As far as terminology (a hotly debated issue!), here's what I do. When kids are very little, I say, "the lady who grew you in her belly."   Once they are 3 or 4, I begin interchanging that with 'birthmom'. 

If I had a child who actually had memories from birth family, I think I would simply say, "your first mom." Whatever terms you end up using, DO talk to your kids. Keep those lines of communication open!