No costumes for my kid

Being that my sister's family is so into Halloween costumes, I usually find myself thinking of what to dress up as quite a long time before October rolls around.  The other day, we tried to get Gavin to try on a costume and he just wasn't having it.  We got out pictures of other people wearing the costume, we tried bribing with candy, but with no luck.  You'd think we were breaking the kid's arm, the way he was screaming.  I've been asking him for a few weeks now if he wanted to be Buddy, the Dinosaur (from Dinosaur Train) or if he wanted to be a dragon, or a variety of other random things.  Every time I ask, he says, 'No, I'm Gavin'.  I even asked, 'Do you want to be a Gavin Dinosaur?' and he still yells no. 

So I've been trying to brainstorm simple dress-up ideas that he might like.  I really think he could be okay with being a train conductor, since he's so obsessed with trains.  However, he refuses to try the hat on.  And yes, I know it's only Sept 24th and we still have a month to find something, but I'm afraid that we might not have a costumed kid this year.  Most parents would be happy about this, but I'm sad we don't get to use one of the amazing costumes my bro-in-law has made. Any suggestions?  I'm getting kinda bummed out.