Input requested for 7yr Old boy and His Feelings about Himself?

Sorry this is long--but:

We adopted siblings when she was 5 and he was 1... in 2003.

A Little background:

She has significant issues and usually captures a lot of attention and remembers everything. He is developmentally delayed about a year and has some significant learning disabilities but the kind of personality that charms. They are biological siblings and at this point have such completely different feelings about everything. It has been difficult for us to deal with their totally different feelings and memories and there are times when they are so conflicted with each other it's nearly impossible for them to understand how the other feels--let alone us watching it all happening.

During the past several months it is clear he has been "putting things together" and it's hard for us (including his sister) to understand where he is coming from with some of the feelings he shares or things he says.

About 6 months ago he started asking many more questions and it took all of us a little while to "Catch" on because he kept making references to his "Baby Sitter" which is odd since he never had one?

Eventually, all of us realized he was switching his words and "thinking" of his biological mother as the "BABY"sitter. Which in his way of thinking made complete sense because we realized he was thinking of the "abstract person" who was connected to him as a baby.

Daddy (they are best buds) decided it might be a good time to sit down with him privately and for them to go through his book with all the pictures and information. He was very engaged with daddy and asked a lot of good questions. Dad said it was a pleasant and positive conversation and he seemed to be happy about talking and seeing the pictures. -- Until he walked out of the den and intentionally said something negative to his sister about their mother...which hurt her feelings and was most likely his intention. (I had two little brothers and they can be like that)

Anyway, his main problem seemed to be about the differences between his mother and me. Our hair color was the big one. And I did some talking with him about his feelings. Nothing big he is so young and it's hard to tell what sinks in.

Last night something on TV brought up Birth as a topic and he said something (didn't really understand) about "I never got born"... Well, big sister jumped in and said, "Oh, yes you did--I know because I was there!" and that just ticked him off for some reason.

Now, tonight comes and the boys were going to play a game and I heard him say, "Lets Play Sorry--I have liked that game since I was in someones tummy!"

I was thinking maybe that came from the One-Up games kids play like, my dad is bigger then yours or I liked Ice Cream before I was born." So, I just asked, a question (can't remember the exact words) to the effect of, how many times do you say that? It was more to find out if that kind of game between kids was going on?

I am a little concerned about what he said next.

"I don't say that to anyone because some people were never in a Tummy!"

Huh? At that point I just let him know--that All babies come from a mothers tummy and that's not different for your friends. You were adopted so you were in your mother's tummy and I got to be mommy when we adopted you.

I left it at that. But, I worry about how he is feeling. I can't tell if he has some kind of loyalty to me or doesn't want to think I am not mommy (get that from some of his other odd remarks) Or if he is still just too young to Get It? Or is he really having so much trouble developmentally or What?

I don't want to confuse him, or make him worry, I don't want him to have any negative feelings about his mother. I am Not sure how much time to spend going over things we have usually taken the lead from the kids.

He is a He...he just turned 7 and he does have some real learning delays, Should we expect him to understand better then he does? Or does this seem to be appropriate for him at this stage?

I would love some feedback from Males I want to respond to him in the best way I can as a mother. I really don't want to trip him up with all this but, it seems so clear he really is dealing with all of this so I don't want to ignore it either.

Thanks for any thoughts...