what do I do

So my boyfriend is a bigger guy. not fat but he's tall and wide. he always says he's fat and huge. but he's not. he has a little bit of chunk but not too much. anyways, I asked him if he wanted to do couples costumes for Halloween and he agreed but everytime I bring up a suggestion he just says that he's too fat for that or he's too big for that and it kind of makes me think if people look at me like that or going to think that about me because I'm a little chunky too. it just kind of sucks because he doesn't want to do anything and he's always putting himself down which makes me feel insecure about myself