Sounds like my husband,,,(only kidding).
Let's face it we all have certain clothing that we like - it's comfy, we like the way it looks, we think we look cool in it.
First - set up expectations in advance. As the week goes by tell him that when he goes to _____ (church, out to dinner, etc.) that he is going to wear nice clthing, acknowledge that you know he doesn't always like to wear other clothing but that he's getting to be a big boy now and he needs to dress and behave like one. Tell him that you know he can do it and you're going to be so proud. Remind him a few times. Kids will usually live up to our expectations.
Consider taking him shopping with you so he can be involved in buying something that he thinks looks cool. For whatever reason kids get these ideas about what's cool based on what other, older kids are wearing. Maybe it was a older kid at church, in the neighborhood, an older cousin, someone on TV, whatever. So give him some control in buying new clothes if you can. Just give him the guidelines in advance. For example my kids know that I won't let them wear clthing with skulls - so after a number of attempts to pick out shirts or sweatshirts decorated with skulls, and me saying no, no, no my son has finally given up. Make sure he knows what you are going to Target, WalMart, Old Navy, etc to look for.
Also - some clothes are itchy and uncomfortable. My kids always have a problem with tags. We cut out any offending tags. I don't need to know what size it is or who the manufacturer is. I need my kid to be willing to wear the shirt or pants.
Lastly - he will grow out of this phase - but into another unfortunately. We need to find a way to let them have some control but also comply with our guidelines. My middle school son will no longer wear a traditional winter jacker becuase it's not cool. So I ended up buying him a sweatshirt with furry lining. He's warm so I'm happy and he thinks he looks cool so I'm happy. It didn't cost any more than a normal winter coat - so we compromised. It is clearly not my first choice - but it will do.
Good luck mama - this parenting stuff presents all kind of dilemmas - we can only hope that we handle it all correctly...!