A pet could help. And are there any cousins you can link him in with more closely?
I know a few only children, and they are fine. I think the main thing I have noticed is not being as used to sharing (space, things,decisions etc), but giving plenty of chances to spend time with others and to share should help that.
And although it is lovely to have siblings, being the only one means you are really special, and good enough to be the only one.
Perhaps off the wall, but have you considered fostering??