This is Ditropan Extended Release 5mgs, the generic drug name is Oxybutynin. This is used to relieve urinary frequency, incontinence and related conditions.
However, the real question here is why your teen had it, and what they thought it was. Many, many times, drugs are sold as other than what they really are, it is possible that because of the 5XL on it, he bought it or was given it as Oxycontin 5mgs or something else. So while this one is nothing dangerous, a few pills taken occasionally will not even make them pee less or anything, if they have normal bladder function. You have to take it regularly for it to have that effect.
However, like I said, we have no idea what your child thought it was and it is a prescription drug, so therefore illegal to have if it wasn't prescribed to them.
So you may want to have a chat and clarify a few things.