Boyfriend has Insecurities

So I’ve been dating my boyfriend for about 9 months now💙👫 and I’ve noticed that he has a lot of insecurities about himself. He is the oldest of 2 boys in his family and used to be overweight, so I’m sure that plays in on some of it...but a lot of it seems to have to do with his younger brother 😩 Their dad passed away a few years ago and broke the family some, but the brothers have always had a strong bond to the point that they even currently live together.💙💙 However, the younger brother always bashes my boyfriends cooking (which is amazing btw), the way he cleans, his choice in women (because I am closer to the younger brothers age), the way he handles things, just everything. And the younger brother always talks about how he gets any woman he wants and always has (which I very highly doubt since he is an ass to most women), which doesn’t seem to bother him now because we are at a very good place in our relationship like I see a great future with him👫💙 ...but it all just seems to bring him down a lot 😩😩💙 he is afraid to cook new dishes because he doesn’t want it to actually turn out bad and be looked down upon. My coworkers were talking about how he is good looking and stuff and he thinks I’m making it up. He is always trying to keep the house clean and even does his brothers chores (he is super organized so he even has chores set out for the guys he lives with).

Like I just don’t know what to do...I have an AMAZING boyfriend and no matter what I try to do to prove it I feel like he thinks I’m just making it up...how do I help make my man realize how great of a guy he is?