How do I get him to trust me?

Okay, so here’s the rundown. My boyfriend is very cautious of me being with boys. Or around them. Theirs one in particular who is my sisters best friends boyfriend he is 16 I am 21! There is nothing going on and I just help him and her hide their relationship from there parents. And my boyfriend knows all this but still gets jealous when I’m around him. Well today I went to visit my bf at work and I had the 16 year old with me and my younger brother. Because the 16 year old was going to hang out with his girl. Well my man got a little cautious. Then he came to my house a little later for lunch. The 16 year old was in the same house playing video games and so were my parents. But my bf got upset. And told me that he thinks something went on with me and this kid. We’re both pretty upset now. What should I do?