B.
It wont come in until it was ready to push the other one out.
Boys usually loose and get teeth in later than girls. Depending on what tooth it is he might not have been due for it to fall out yet.
http://www.medicinenet.com/script/main/art.asp?articlekey...
My daughter had one of her front teeth knocked out when she was 4 and it didn't come in for two years.