Get used to it:) I have a two and three year old and I seriously thought they were going deaf because they never responded to me. Then two things happened...one, a friend told me to whisper "candy" and see if they hear and respond to that..(they ALWAYS do!) and then the second thing was that when I asked my three year old if she heard and understood me she looked at me and said "Yes, mommy, I hear you, I just dont like what you are saying!!"
So, my point is that if your kid is really involved in something (a toy, a book etc) they are probably not going to respond to their name because you are "interrupting them". We dont like to be interrupted when we are in the middle of something, but we have learned to be polite and acknowledge someone speaking to us. A 13 month old just hasnt yet. If he usually responds and there are no other concerns, I would think you are OK, but check in with your doctor if you're not!
Good luck!