Had a lot of very difficult questions over the years, as i suspect nearly all of us adoptive parents and guardians have had, and you have all my sympathies, it's very hard
From the sounds of things, I think you're already handling it very well
I think when talking about the past and birth parents, it's important to be honest, whilst also recognising that young children need to feel wanted and loved. So the answer to "Does my birth mum love me?" would usually be either "yes, she loves you very much, but she can't look after you because xyz" OR maybe if appropriate, "I don't know for sure, but I think she did. You were a wonderful, lovable baby "
Honesty, not demonising birth parents, although recognising when appropriate that people can make bad choices/decisions and not lying if the background is very difficult, and emphasising to children that they are lovable and wanted very much, is what we should try to do. And have an open door policy and let our children know that birth parents are an open topic of conversation and they can ask you anything at any time and you will always do your best to answer.
I think you are right in trying to get across to your DN that he isn't alone and this is also something which happens to other children. My older children have really liked meeting other adoptees who were in care, because unlike at school, they aren't 'different', but they fit in with these other children and share some of their experiences.
If we do that, I don't think there is much more we can do. If a child is asking a lot of questions over time and is sad and unsettled, that doesn't mean we are handling it badly. Some children ask a lot more questions than others, and some children find it much more difficult to 'come to terms' (not sure what the right word is) with their background, and to believe in their own self worth and lovability, than other children do. It isn't a reflection on the guardians/carers/adoptive parents ability to handle questionning, it may well be the childs natural personality and way of thinking. We all hope our children will have good self esteem etc, but we have to accept that we can't make it all better and we usually have no magic words to help a hurting child feel much better.
Also, most children have certain periods in their childhood where they think about their past more than at other times. So you may get months with a lot of questionning then very little for another couple of years then suddenly a renewed desire to know more and question more for months etc. That's very normal.