I'm not sure if the worms you describe are Roundworm or Tapeworm. Benson could have become infested in lots of ways. Fleas are linked to the tapeworm cycle, dogs that catch and eat birds, rabbits, mice etc. may contract tapeworms from them, and worm larvae in dog poo can burrow into the ground and stay there for a long time.
My 2 were wormcounted last year and Louis was worm free but William had Roundworm and Tapeworm. He's the one that is always digging soil and nosing at things when we're out walking, so I wonder if that's how he ended up with them? I wormed them both to be on the safe side. I think if I were you I would definitely worm all three.
Perhaps try a different wormer this time, like Milbemax?