I am getting married next year. I have two brothers. One brother is technically groomsman but doesn't particip?

My brother is technically a groomsman but doesn't want to participate. He has been negative. He doesn't want to join everyone in suit shopping. He states that he has enough suits. He declined joining the group anyway without a reason. He doesn't like talking about the topic. He is upset that my parents are paying for 1/2 the wedding (the other half will be paid by my fiancee's parents). His wife declined to be a bridesmaid. He does the bare minimum. As far as I know, he does not have an issue with my fiancee. Why is he acting like this?