My bf pretty much accused me of ?

My boyfriend and I are at uni together, and my male cousin came up to see me the other day. This is the first time the two of them have met, he's my oldest cousin (he's 27, my bf's 21 and I'm 20) and probably the member of my family I'm closest to (I've lived with my uncle's family since my parents died when I was 7, and he's always been like an older brother and looked out for me). My bf seemed a bit off while he was around, and when my cousin left he asked me what the "nature of our relationship" was. I really couldn't believe that, and I'm still really angry and offended. My cousin and I act close - kiss each other on the cheek, cuddle, etc. - but we're FAMILY. My bf was also really petty about him, saying that he was acting really patronising (my cousin already has an established career in business) and said he acted really threatening towards him. I'm not sure whether I should excuse my bf or not... This behaviour seems really out of order to me... Neither of us get jealous very easily so I'm shocked that he suspected my cousin of all people. Should I reure him, or wait for him to apologize?