In "Through the Tunnel," how is the mother an obstacle to Jerry's initiation?

I believe that the "initiation" that your question mentions is Jerry's initiation into adulthood.  Or in becoming a man, whichever you prefer.  I don't feel that Jerry's mom is too much of an obstacle.  To her credit, she lets Jerry go off on his own on most of the days.  She absolutely could have forced Jerry to stay at her beach within sight of her.  She did do that for one day, which was a minor obstacle, because it prevented him from further attempting the tunnel swim.  She also is an obstacle in the same sense that a lot of moms are to young teenage boys.  Mom, and being seen with mom, simply is not "cool."  Jerry wants to be cool and independent, and he can't do that if he is forced to hang out with his mother.