My five year old can't remember names?

My five year old son just started kindergarten and I've noticed he's had trouble learning his letters. It seems like if you teach him something, it's in one ear and out the other. I had never really thought about it before, but I've noticed that he also has trouble remembering people's names. I am divorced from his mother, but she says he even has trouble remembering his own older sisters' names. Instead of calling them by their names, he will call one the blonde one and the other the dark headed one. He knows their names, but can't seem to distinguish which is which. Likewise, I've noticed that he can't seem to keep straight the name of my sister (his aunt) and her daughter (his cousin) and will always ask which is which. Also, he can't seem to remember the names of people he is friends with at school. I've asked him before who his friends are and what their names are and he always says he can't remember. I took him to a school event and there was a little girl talking to him and calling him by name and I asked him her name and he said he couldn't remember. This concerns me. What do you think this is? And is there just cause for concern?