Most Helpful Guy
I would say start a relationship.
If you're best friends it already means that you like each other,
You like being together and hang out.
If you're into him just start a relationship,
I think it would also make your relationship much better,
You will get to know each other better.
If you're already best friends why not to make it THE best friends :).
In my opinion just start a relationship.
Hopefully you will be very good friends and it won't effect your friendship.
Don't push it.
Have a great day,
Hope I helped.
FYI : If you have anymore question or updates for the case just comment, I'll be glad to help :)