I have a son who is 9 1/2 years old. I have sometimes thought about having another little one, but am almost 40, so realistically, it's pretty unlikely.
1)A small preschool with older and younger kids is great.
One of the best things for my son was that he was in a wonderful home preschool when he was young. He was able to have the experience of being one of the 'littles' and following around the bigger kids, and then being a 'big'. He learned to be so patient with smaller kids from that, and now he is a great 'big cousin' to my brother's kids who are all younger.
2)Get him a friend with a lot of brothers and sisters!
He has sometimes wished to have a little brother or sister. But his best friend is one of 4 children and although he loves going over to their house to play, it always makes him appreciate having his own room and stuff to himself and his own time alone to read or whatever- that is in short supply at his friend's house, lol!! In fact, his best friend loves to come over to OUR house for just the opposite reasons that my son likes going to his! The grass is always greener...
Having only one child has made things easier financially, etc. for sure. He has been able to go to sleep away camp, go do special things with us, etc. I know we would not be able to afford to do so much with more kids and I think he appreciates that too.
Of course, that also means he has more chores and things to do around the house. But he is a very responsible child and although he is social and has a lot of friends and is very self-confident, from an early age he has liked to read or play on his own. He can entertain himself for hours and I think a lot of that comes from being the only kid around from a young age. He is a gifted student and was an early advanced reader, all on his own, with no pushing from me. I find him to be more responsible and mature in general than a lot of his friends, although I don't know if that is from being an 'only' or not.
Now that he's older, it just doesn't seem to be an issue at all for him one way or the other. He is happy and well adjusted and knows that I love him 'best of everything', without being spoiled. I know a lot of only kids get spoiled, but that is because of the parent, not the kid, IMO. I think with an 'only' you have to be careful NOT to smother them all the time with attention- just give them some space to do their own thing, play on their own, etc. My son is a really amazing, mature, thoughtful, funny, smart kid- I'm sure yours will be too!