How could i become best friends with someone who already has one?!?

So this is really hard to explain and it's a really weird situation I'm in but i need your help. Alright so i decided i needed a best friend that i can do pretty much everything with, I had one but then i moved schools and he started acting all weird. So the problem starts when the kid i want to be bestfriends with already has one, right now me and my friend are good friends but i want to change that to best. We pretty much have everything in common. I wish the three of us(me, the kid i want to be bestfriends with,his bestfriend) could be all bestfriends .Any ideas on how i could be bestfreinds with this kid, im tired of not being able to have that one person you can tell everything to or do everything with, i need that support to be there.How could i become best friends with someone who already has one?!?
Expand you horizons you don't have to be best friends just with one person get to know and spend time with different people that what I do. I have friends that have different interest so I do things with different ones depending on what the occasion is. If you do want to be friends with these guys then just start asking them to hang out. Tell your friend that you want to hang out with him but that it will be cool to bring his friend along.