Honestly
with words
Then just be friends with him.
Okay well first is your guy friend bi aswell? If not dont go onto him. Youd just freak him out if anything.
If you want to stay friends you do nothing unless he is also Bi and fancies you "That way" which I doubt he does.
When he starts liking girls too.
Only sure way is to ask him but he may not tell you the truth.
there is such thing as to much open minded. if your willing to be in a relationship with another man you are bi. but if you've changed your mind now and don't want to be in a relationship with a guy then tell them that and they might believe you.
ask him or one of his closest friends
you can tell if a guy doesn't want to be friends with because maybe hes avoiding you or hes not speaking to you
if they are real friends just tell them and they should believe you.
become friends with the guy.. because you cant have a relationship with out knowing him
first, ask him what he thinks of girls that are bi. if he doesnt mind then let him know. if he does mind then just tell him you wont do anything with a guy or girl to loose his trust:)