"My Happy Birthday" - the continuing saga

We went to a 2nd birthday party for a little boy last Saturday.  Nathan was very excited to sing Happy Birthday to Carter all day long, until we got to the party when he completely clammed up.  Tonight I was out running errands with Nathan and Jay when Nathan started singing Happy Birthday to Jay-Jay.  This has been a dilemma but Nathan seems to be grasping the concept.  Even sweeter, however, was when we were looking at the Christmas lights and Nathan said Happy Birthday to Christmas.  I then told him that Christmas is actually the time that we celebrate the birth of baby Jesus, so Christmas is His birthday.  He then started to sing again, this time, "Happy Birthday, Baby Jesus! Happy Birthday to You!"