Don't panic M. - he will survive!
I was an only child. However, not sure I ever knew it! I always had friends over for sleepovers and any family vacations, I always got to take my best friend with me, wherever we we went. I now have 2 children, and I have had some interesting conversations with friends. When I said - "how do we now take two children AND their two friends on vacations - now we will have 4!" My sweet friends laugh and say - when you have more than one kid - their sibling IS their friend on vacation, they don't get to take friends! I now understand how lucky I was growing up! My mom made sure I always had someone to play with, and lucky for me, it was always someone I chose to ride 10 hours in a car with and with whom to share a room - not a sibling! (Probably was a happier trip for the parents too!)
When I was older, we hosted an exchange student for a year, which was an amazing experience, and did give me a chance to enjoy the sibling experience. It was a novelety to have someone day in and day out to share time with, but I did realize how lucky it was that I didn't for the first 17 years!
Now that I have two, I realize I often expect the older one to entertain the younger one, and the thought of having two playmates over for the two kids for the afternoon sounds a little overwhelming. However, I think back to my childhood and how wonderful it was that I could always have playmates over.
So, your son probably wants a sibling because at the 2-5 year old range, it seems a lot of people are getting pregnant with their second in his preschool class. However, as far as his growing up lonely, you can make sure that he has a constant parade of friends that can spend time with him, teach him to share, fight with on family vacations over the best bunk, etc. And the best thing is, when he is tired of being with them - they can go home!
It may take a little more social planning on your part to arrange playdates etc. but he will soon appreciate being only child when his friends come to play and tell him how lucky he is not to have younger siblings tearing up his things and trying to be in the middle of everything!