No it is not be cause he is an only child, it is because he has not LEARNED to take turns. My sister used to be like this when she was young. Our daughter also had a very good friend (she has a brother)that was like this at this age.
I noticed my sister and this little girl are very competitive. Which is fine, but I think they need to be reminded that being in line is not a sport. Being first to get something or be somewhere is not a sport either.
My sister and this little girl tended to be self absorbed so maybe your son needs to know not everything is all about him.. Gosh knows my sister is still dealing with this and so is my daughters friend..
Model the acceptable behavior. When you all are out and about, allow others to take your place in a line. I do this at the grocery store. If I notice a mom with a child or children, or elderly person pull up behind me, I allow them to check out in front of me,if I have time to allow it. At a restaurant do the same.
If you are at an event, you and your son help serve to others (elderly, moms, young children) before you serve yourselves.
I always taught my daughter to ask a hostess if she could help pass cake slices, pour more iced tea.. We always "allowed ourselves" to go last. This way we had a chance to help our hostess. He will see that there is plenty to go around even if he is the last served.