Take a look at the sentence again:
When Matt shaved his head, his mother worried that he had joined a
cult the real reason for the bald head,
however, was that Matt could get more attention and sympathy from girls
who thought he was sick with a dread disease.
For a sentence to be a comma splice, you must have two complete sentences joined with a comma. In the example above, there is no comma. His mother worried that he had joined a cult is one sentence. The real reason for the bald head ... begins the second sentence. Because only empty space exists between the two, you should call this error a fused sentence.
Remember that both comma splices and fused sentences are major errors. They make your reader think that you cannot write a correct sentence. Because the sentence is the most basic building block of a piece of writing, comma splices and fused sentences make you look like an amateur!
To fix the problem above, you could put a period after cult and begin the with a capital T.