Does a bechor needs to have younger siblings? Is he still be a bechor if he happen to be the only son in the family?
Obviously, he will inherit everything so the halacha of "double" portion does not apply. However, the halacha of pidyon haben (redeeming the first born) does apply as does the custom of fasting on erev Pesach.
The halacha as given in the Torah says that pidyon haben is required 30 days after the birth of the bechor. In most cases, a woman will give birth to a single child rather than a multiple birth (such as twins or triplets). Obviously, in most cases, at that time he has no siblings. Thus we see that the halacha applies whether or not the bechor is an only child.
It is customary for all firstborn males to fast on Erev Pesach in order to recall the tenth and final plague which God inflicted upon Egypt – the death of the firstborn.
 Tur 429, 470, Mishna Berura 470:7
Note that while the first born is a child, the father will fast for him (as does one of my sons for his bechor).