I'm so sorry for your son's loss, and for the kid's family... how tragic.
FWIW, I think space would be a good thing, especially as all his mates are going to be there to hold him up, so to speak. I attended the funeral of a friend who was killed in a car accident at the age of 15, my mum came with me for support and *she* was so sad she was throwing herself at me, hugging me to death and sobbing her eyes out... I kept asking her to just leave me be (I'm the type of person that kind of withdraws in situations like these - I don't like to be touched or made aware that anybody else is around, I just want to disappear into myself and process what I'm going through) and stop touching me, but she wouldn't - apparently my tears made her want to comfort me and she just did a terrible job of it (not to mention making a frickin' scene in the church by getting upset with me when I slapped her hand away as she tried to grab mine).

When DH's mum was killed last year, BIL hadn't yet turned 16, and we were very worried about how he was going to cope at the funeral. We all went as a family, obviously, and he sat with us at the ceremony, but as we walked out, his mates kind of gathered around and whisked him off to a quiet corner of the crematorium grounds and when I checked on them 10 minutes later they were laughing and hanging out as usual, I mean the general air was much more sombre than usual but he seemed to find more comfort in being with his mates, and attempting some normalcy, rather than being around us and his grandparents. And when it came time to head back into town for the wake, he elected to be dropped off at a friend's house instead.

So I think if you want to attend the funeral to provide support, my advice would be to do it from a distance - I know as a parent you just want to fix the hurt and be the shoulder to cry on, but teenagers seem to be biologically designed to reject their parents and find comfort from other sources, usually their friends - and in this case, your son's friends will be experiencing the exact same emotions and will have that as a point of relation, kwim?? 'We're all in this together,' sort of mentality.
Go to the funeral, for sure, but just let him take the lead - if he indicates that he wants you nearby, go with it, but if he does the ol', 'I'm pretending you don't exist' then just do it like that. I'm sure neither you, nor you DS, will be able to predict exactly how he's going to react until it happens, so being there in case he needs you is a good idea, but don't be surprised if you find yourself unneeded lol... sorry, that sounds harsh, but yeah - kids tend to band together in these kinds of situations and get emotional support from one another rather than 'sooking to mummy', kwim??

Good luck with whatever you choose to do. I hope your DS's broken heart heals soon and that he and his friends get the support they need during such a confusing time. And I hope his friend is now resting in peace.