The phrase "something like his old clothes" is a simile, a comparison using the words "like" or "as". The irony is that he is wearing what used to be his old clothes, but they have become so tattered and torn with time spent on the island, that they no longer look the same. Jack has begun to look more like a savage than a choir boy.
Also it is important to remember the symbolism in that as well. The boys have changed since coming to the island. The clothes represent what they once were. The author is reminding the reader of the sharp contrast of what once was and what is now. By putting on the clothes, Jack also shows us that he remembers what he was and it may foreshadow the change in his heart which happens towards the end of the book.