Unless you are planning on showing the dog I don't think it's anything to worry about. If you are planning on showing then you should contact your breeder regarding the bite. Without actually laying hands on the dog I really couldn't tell you if the under bite will get worse or if it is even an under bite at all. My male has a slight under bite and it does not affect him in any way.
You could also try taking him into a vet, they can actually touch and see him and tell you if it's an under bite and advise you as to whether it will get worse or not.
"i can explain it to you, but i can't understand it for you."