I would wonder if he forgot about their existence or if he forgot the relationship for a minute. Or if he feels his family relationships are more important.
My dp has a dn. We have quite a large age gap and dn is actually only a few years younger than me. I certainly do not see her as a niece in any shape or form. She was born a long time before we got together. Dp obviously does see her as a niece.
We attended her wedding, we see her fairly often, I would describe her as close, but not my niece like my siblings (small) children are dnieces and nephews. She has children similarly aged to mine. I totally see them as my children's cousins. But they are 2nd cousins obviously.
We do tell our children how they are related to people, but to be honest, they do not care, or even seem to retain the information.
We go over to Ireland to see my family and they are unsure who they are related to and how sometimes. There are loads of us to be fair, and then also my friends' children that we see a lot of. My son says Ireland is just full of cousins and love and I love that he has such a sense of belonging. Your child will define their own relationships. I would relax and let them.
My son's favourite "cousin" is actually the daughter of a v good friend of mine who lives in Ireland. No matter how short a trip we make, we have to see them every time, or he is really upset. My wee one does not care yet. I think it best to facilitate their own ideas.