One other random thought re: making friends in general (not the pre-K)...
Have you tried matching him up with MG+ kids 1-2 years older than he is? MG kids are usually easier to find than HG+ kids because statistically speaking, there are just more of them.
We had some lasting friendships formed that way when DS8 was aged 3-5 years. He's still good friends with a boy he met in pre-K who is MG-HG and a year older, even though they haven't gone to the same school since then, they don't live in our neighborhood, etc.
The GT component helps with the intellectual peer part, and 1-2 years isn't such a wide range that kids won't play with the younger child.
Maybe irrelevant, but hopefully useful!
