Is the name close to your actual name, does it start with the same letter or sound similar?
Maybe you remind him of a friend or previous girlfriend who was called that name.
How many times has he does it? I could understand if you'd been together for years with kids as mr mum often has to wait till I go through all the kids names before I get to his. BUT at 23 and seeing someone for 6 mnths-once is forgiveable, twice just about tolerant but more than that-bye bye!!
you should pay close attention. he talks to that person constantly. if it is a female, then you need to find out what kind of relationship they have. it might just be a very good friend but you never know. investigate. it seems that your intuition tells you that there is need for concern - maybe but find out.