Until then, understand that she loved him enough to want the best for him. It was the most painful thing in the world for her to give up a child out of love. And that he should understand and not make things harder for her.
Out on a limb here.
Not all kids will understand or have the ability to understand why they were given up, out of love, especially, particularly, children with attachment issues. Telling a child this is and hoping they will understand, well, I don't think he should feel that it's his responsibility of making it harder for the birth parent.
I agree that the mother has the right to privacy and her decisions are foremost in a situation such as this, these were her choices. I also agree that children need to be told 'no' in many situations because explanations can be messy or can cause more damage.
But I also know that when a child is told they are given up for their best interests, or out of love, that when a child repeatedly asks the question "why won't my mom/dad see me"... that giving no explantion or a less than honest explanation, that it can really damage the child. Saying it was because they were so loved - that doesn't make sense to a child. Children with attachment issues, I believe, more so.
They don't all understand. Certainly, the stigmatisation (if that's a word) that children feel who are 'given up' is huge for children. Each child will react differently. Each circumstance is different.
It isn't the child making it harder for the parent. They are the children. They shouldn't feel responsible for making it harder for the parent by asking to see them. The answer can still be a no - but I think the professionals involved with the issues Mo may have - maybe need to have some input.