Firstly, your little boy sounds very bright! I wanted to chip in here because I was exactly the same at primary school. All my friends were going to the local comp - including my Best Friend Forever (where is she now?!) - and I couldn't imagine a world where they wouldn't be the centre of my existence. The turning point for me was going for an open event at our local SS where I suddenly realised the extent of the opportunity I would be turning down - because we got to make goo. That is about as sophisticated as the mind of a 10 year old gets: "Mum I don't want to go to that school; how will I ever live without my friends, who are part of my very soul?! Oh, hold on - you can make goo at that school?! Who knew?! Bye bye dearest friends - I'm off to a school where I shall immerse myself in daily goo-making experiences!" I didn't ever visit the comp, in fact - and I think that was probably a good thing; what if they had let us make bracelets? Or explode something in the labs? That might have blown the goo out of the water.
Fortunately, I didn't ever learn what happened at the local comp, and toddled off happily (a walk, bus, bus, walk journey). I made some wonderful friends - people that are still my friends today. And, in fact, it was one of the best things that ever happened to me. I entered a world where the sky was the limit - where we were taught we could be anything we wanted to be.
It sounds like its a hearts and minds battle you're fighting, too. So, my advice would be, firstly, to gently remind him - occasionally - of the benefits of the education he could get. Explain your point of view. As you say, he is 10 (9?) and can't see far beyond the issues that are important to a 10 (/9) year old - but I think you can try to help them do that. Show him the difference in results between the two schools, in university destination, and explain what that means for him and his future. Ask him what he wants to be (astronaut? doctor? fashion designer? the troll under the bridge from the Billy Goats Gruff? These are some of my DD's current career options) and show him that this school can help him be whatever he wants to be. It might sound a lot for a littl'un but I had these sorts of conversations with my daughter (now year 6) when she was going through a "lalala, this 11+ is a total lark - I don't think I'll bother marking the paper at all!" phase. I explained that I wanted her to have options, and that all I cared about was that she could walk away from the tests knowing she'd done them to the best of her ability. Come what may, she would always know that she was someone who did her best. I told her that the only thing she could ever regret was not trying. That if she tried, and didn't get through on that day, in that test, that she would always know she was brilliant in her own right, and that most importantly, she's tried her best. It worked for her, rather miraculously I must say.
Another thing that helped was making some friends on the insight day (the one with the goo). Just knowing I could see those people again was a carrot. Could you try to get him friendly with some other boys who are doing the tests? Maybe arrange a meet up with some of them from the forum if you can? Or enrol him in a group 11+ course if they have them in your area - I used to run them and the effect (if they're well run) on the kids is brilliant - just meeting others in the same boat, that you might bump into on the day of your test, makes them feel part of something. It's amazing the effect that a friend can have. Almost as powerful as goo making experiences. Vet them carefully, though - some of the courses can have the opposite effect.
Finally, if there is any opportunity for an insight day, get him along! You never know, they might make goo.
Oh - and re: the purposefully failing - I would say remind him that the tests are very difficult to pass and that lots of people don't - but that he could regret it if he doesn't give himself a shot at it. If you can persuade him to do his best, you can then make a decision, together, about what's next.
The friendship thing is probably the most powerful, though.
Sorry for the essay and I hope I don't sound preachy! Because of my experiences, I totally understand your DS's point of view. Really hope you can get through to him on this!