Since Patrick IS evidently dead, Matthew is heir. His son, is next-in-line because he is the male heir to Matthew. If the boy were to die without an heir, they'd have to backtrack through the line again, to find another Crawley male. Since this is all through the male line, it is most likely it would be someone named Crawley.
Unless that person, or someone earlier in his line, was "adopted" by another family. He would still be in the male line of succession. See Pride and Prejudice
's Mr. Collins being heir to Mr. Bennet's estate, although it was entailed to the male line. Mr. Collins' father and Mr Bennet were cousins and had a falling out. I don't remember why, but it might've been due to just such an adoption--or a fight over Mrs. Bennet (lol).