Well man, my suggestion is hard to swallow but it's the voice of experience: just cutting him loose and get him out of your life.
I'm 60% 40% bi. Slightly prefer men cause of the ease of sex and the excellent blow jobs, but I'm very female friendly. My point is I've messed around with guys and if your cocksucker can't keep his mouth shut about sex, your personal life will get very complicated.
A few years ago a gay sex partner couldn't keep his mouth shut, and he told a female friend, who told another female friend, who was married to my best friend of 10 years who is 99.5% straight. The wife never liked me, and really laid into him what a nasty faggot I was. The friendship wasn't recked only because my buddy already knew for years that I swung both ways and had gotten comfortable with it. He told his wife to get her nose out of other people's business and that was that.
My point is if your "buddy" is already gossiping about the size of your tool, I'd bet $100 he's also passing along how hard up and horny you are, and how easy horny straight boys are to seduce. Once the dirty deed is done, there will be a report on that too and word will get around.
I'm not discouraging you from getting male head. Men are vigorous cocksuckers. I didn't get my first male BJ till I turned 30 and it was fantastic. But I'd pass on this one. My gut tells me it's more trouble than it's worth.