As you know they live in damp soil or sand so he could have picked them up in any park, the grassy area at petsmart, crossing someones yard corner on a walk etc. Hookworms are the reason many lakes,beaches and parks do not allow pets. A passing dog could also have left it in an unfenced area.
Stool should be picked up to prevent reinfecting.
The panacur should be repeated at about 2wks. You may want to check another sample 2wks after the 2nd round.
The info I found online says they hatch in 18 hours but then the larva can live "for many weeks without food"
Hookworms
I have not had them but did find where it can cause loose bloody stool.