Bring out the costume and lay it down next to him and let him connect with the costume. If your dog goes on the costume and stands on it or starts sniffing the costume, it generally means that he likes it. This doesn't mean that he'll wear it but it's a good first indicator that you stand a chance at getting him to enjoy wearing it. Don't worry if he doesn't approach the costume, as it may take some time. Remember to reward him with treats when he takes a positive action towards the costume.