I hate to say this but I don't like how my boyfriend dresses.
Really? Why don't you like how he dresses?
He wears old clothes and they make him look sloppy or like a homeless person.
Its no problem to talk about how your boyfriend dresses to him. He may decide to swtich up his attire if he knew how much you disliked it. Open communication is paramount to a good relationship.
I don't think he'll listen to me, he's a free spirit.
If you don't think he'll listen to, why don't you take him shopping and help him pick out some new clothes that he likes. If he sees how much you like the clothes, maybe he'll wear what you bought him and stop wearing his previous clothes?