Unfortunately I think it may be a case of "build it and they will come" 
From talking to friends, it seems that many kids this age don't like unstructured socialising with other kids they don't know. It makes them nervous. They are becoming more self-conscious and starting to mind the prospect of going along to an event and finding no one to play with (sorry, I mean "hag around with") and perhaps worrying more about how they will look if they are on the outside of a group who already know each other. They don't want to be seen sitting by mum with no friends. Even my 12yo - who's outgoing and confident and at the end of the day can find something in common with just about anyone - has begun to quiz me over details of any group I propose to take her to, and specifically Who Will Be There That I Know. This is also an age when older kids are sometimes allowed to opt out of being hauled along to younger siblings' activities, because parents may feel they are old enough to be left home alone if they really don't want to go, so they become less visible for that reason. And then some of them do already have a few friends because they have kept up with former school friends or friends they've made at mainstream activities.
Unless the secondary schools in your area are utterly brilliant, I expect there really are a lot of HE kids around your son's age. The problem is finding them. Some people in my area asserted confidently that there were very few secondary-aged HE children in the area, because they didn't see them at the groups. It turns out that the place was crawling with them, they just didn't come to the activities set up for younger children. As soon as a few parents had started being more proactive about setting things up that they would like, a social scene began to appear. Among the people I know, it's still the case that preteens and teens spend a lot of time in little informal groups with their existing pals. However, there are now ways for them to meet each other: a Warhammer group, robotics workshops, snowboarding, fencing.
You might try posting a lonely-hearts type message on your local email list, which could uncover one or more people in the same position as you. It will probably then be a case of you and another parent dragging anxious and slightly reluctant kids off to meet each other ("but what if I don't LIKE him??"). You can probably tempt them along with an activity where the focus is not too directly on socialising: bowling, karting, something like that. Perhaps post to the list and say, "Are there any over-tens who want to meet up for bowling? Email me and we'll fix a date."
Someone on one of the home ed lists remarked that it feels strange to us parents that kids of this age appear to need more parental assistance with making new friends than they did when they were younger! You can't just chuck them together in the park anymore.
Good luck, I know it's hard.