How do I tell my sort-of boyfriend that I'm gay?

First off, I'm asexual, and I've identified as biromantic for the past couple of years or so, but for the past several months, I've been questioning whether I still like guys and whether I ever really liked them at all (I know I do definitely like girls).

A good guy-friend of mine who used to go to the same Stagecoach (a performing arts sort of club) class as me asked me out a couple of weeks ago, and I said yes, since we're good friends and I did used to have a crush on him. He said it doesn't have to be romantic as I've told him I'm ace and questioning whether I still like guys, but I think he'll probably expect it to be like that at some point. I saw him on Saturday after this winter's Stagecoach production, and realised later when I got home that I don't have romantic feelings for him, and I'd actually rather not have a romantic relationship with him. I felt really terrible because I think I might have been leading him on to believe there could be something more there, and now I know I don't want that, I'm not sure how to tell him. I think another worry was that he would probably at some point want the relationship to go further into the physical/sexual aspects- though he knows I'm ace and accepts it. I know I don't have any sexual desire for guys and would not want to participate in any sexual activities with guys, but I think I might consider it with a girl. That same night, a good girl-friend of mine who just left Stagecoach told me she's bi, and I a had a crush on her for a while- she knows this guy too, so I might try asking her for advice (also I think there's a possibility she might like me).

How would you guys advise me going about sorting out my relationship with this guy? I want to tell him that I'm pretty sure I'm gay, but I still want to be friends, cause we are already good friends and I don't want to upset him or lose him altogether.

Thanks in advance