The word "prodigal" means someone who wastes what they have, someone who is very wasteful. From that, you can see which son it was that was prodigal. It was the younger son who is referred to in the "title" of this parable.
If you remember, it was the younger son who wanted his inheritance right away. He went off and he spent the inheritance in really stupid ways -- he totally wasted it. Meanwhile, his older brother stayed at home doing everything just the way he was supposed to. So he was definitely not prodigal.
Prodigal means: "spending money or resources freely and recklessly; wastefully extravagant." In the parable the prodigal son was the youngest of the sons and he runs away and takes his inheritance and wastes it.