oral question (69)

So my boyfriend of 8-9 months asked several times if we could 69 it. I've always said no bc I have IBS and I'm always afraid that I don't know when I gotta go or fart or something. I don't want to do that in his face. I told him after the last time he ask that if my doctor appointment went okay with my gastro. if think about it but since the the appt. I have to go for an endoscopy. I don't know what to tell him or what. maybe this is all just a low confidence thing. If anyone can help or give tips it would be much appreciated.