Usage text messages

So I was looking at my son’s phone bill this evening and it says he has sent over 500 messages. I go to look snd the number are showing up as my number but I know I haven’t texted him. However looking back to last months bill it says the same thing: my numbers at times I know we weren’t texting. Does anyone know how to find out who he’s talking to? And how to get it off my number and show the correct number?