This might just be his personality developing. Usually around this age children tend to develop their own likes and dislikes. My mother told me I was so strong willing and I started dressing myself at age 3. She said every since I was little, I knew exactly what I wanted and how I was going to do it and there wasn't much else anybody could say about it.
My kids are the same way. My 4 year old is the spitting image of my personality. We think in the same way. He has been picky about clothes since he was born. He had sensitive skin, so he couldn't have snap on clothes. He never liked overalls or elastic pants. He had to have sports stuff on all his shirts and he would only wear jean shorts and jean pants. He pant legs and arm sleeves have to be folded just so. He's just very organized and likes everything just right or he has a very difficult time moving onto something else.
My mom, Dad, brother and I are the same way. People feel unstable about the things that go on around us. There are so many changes we see in schedules, foods, new friends, family member shifts, school, careers, etc. We try to find small things we can organize or prepare so that we feel we have control over our lives. Perhaps I can't control the busy day I have and the people I meet, but I can control the way I dress. He may have more of a controlling nature about himself, which is fine.
My family has a tendency to be excessively neat and organized with things around them in an effort to mentally control stresses in their lives... at least that's my psychological assessment. Being choosy about clothing, food, toys, etc is a normal part of life in creating our own independence and stability. I believe when it becomes a problem is when those practices inhibit mental or spiritual growth or deter us from trying new things or meeting new people. I don't see choosing your own wardrobe as being a destructive behavior, so I wouldn't put too much emphasis on that without other symptoms of OCD. I think as parents we need to choose our battles, and battling over clothes probably isn't very high on the priority list since your child seems like a wonderful and happy child.