Oh that's so sad, my condolences.
If you can, hug your boy. He will need his mum.
Let him talk about it, and about his friend. You don't say the circumstances but it's likely to unsettle him and he might be feeling rather vulnerable.
My DS lost a friend (not as close) to suicide when they were 14. It was very hard and it affected my DS more than he let on. Make sure your DS talks (to you or someone else) honestly about how he is feeling, whatever the circumstances of the death. Make sure he has another friend to lean on too and encourage him to open up.
I think his generation are pretty good about celebrating their lost friends so encourage him to set up an online memorial /charity fundraiser of some sort and for them to keep in touch with the family of the lost boy. Attending the funeral is important too, so help with logistics/travel costs of that if needed.
WhatsApp is a really good way of keeping in touch with your DS too. I send daily messages to my DS, and when he's going through a hard time, gentle messages about the inconsequential stuff like pictures of pets etc can help him to feel the love and security of home, to help give him strength to navigate his grief.
I wish you and your dear boy well in navigating through this very sad and difficult time. 