Skip the Hello--How are you today?" It's too long, I think. Def do the greeting: Hello OR How are you today, Good day, Good afternoon, etc. Would you like to support the Cub Scouts by purchasing some delicious popcorn?" If they say no, he could perhaps add, "We have many different delicious varieties."--or "It's a very heart healthy snack!"--etc...You get the drift.
And/or just have them be their cute li'l sweet, Cub Scout selves and people will buy the yummy popcorn because they are SO adorable AND the fact that BSA is a great organization! Make sure he smiles at everyone and are very polite and they look nice, representing the BSA!
He will do fine! Let us know how it goes. My son has done it 2x but he's not home now or I would ask him what he says.
One last thing! No matter what the person says, have him say "Thank you" to everyone he asks even if they say no.