Relationship Help

So a girl kissed my boyfriend trying to get her point across while arguing with him about why he should have chosen her instead of me.. ( i was not there ) He has really bad anxiety so because he was afraid of my reaction his sister told me what happen. Honestly what I think about the situation is that he needs to man up and talk to me himself but his sister tells me he is just afraid I'll break up with him, he has not texted or called me in three weeks! I won't call him or text him first because I feel like it's something he needs to come to me about so at this point I'm just wondering if he's ever going to talk to me about what happened or if he's just never going to talk to me again I really just don't know what to do at this point.... I'm 17 and he is 18