Dear N.,
How refreshing to see that this young boy had taken an initiative to contact you about doing some chores in exchange for some money. It is unfortunate that he seems to be slacking in the responsibilities, but not surprising.
As difficult as it may be -- you do need to talk to him and help him to understand what your expectations on for the performance of the job and for his behavior.
Start with the positive and really reinforce his initial approach and how that was terrific that he wanted to work to make some money and that you want to continue to have him do the work, and therefore you want to help him to improve on his performance and his behavior toward you and your husband.
If you don't already have it done -- write out the specific responsibilities of taking care of the cat litter. I would say just keep it simple in a bullet format on an index card:
1) scoop out all clumps in the litter box and put in trash can
2) add about "xx" clean litter to the box to refresh
3) sweep all litter around the box with broom and dustpan
4) tie up the trash bag and carry out to the garbage can.
etc.. -- you get the idea.
Once you go over this with him perhaps tap it to the area where he can see it and remember to do all the steps when he comes over to do the litter.
Maybe also give him an incentive to follow your expectations -- if he does a good job for a month, he will receive an extra "xx" bonus (the bonus could be money, toy, mcdonald's gift certificate,...). Or maybe you surprise him with the bonus after a couple weeks just to keep him motivated.
Likewise, make sure you address the behavior. Again, keep it simple, "XX", it is not acceptable for you to talk to us in that tone." or "XX, it is not acceptable to use those words (or talk to us disrespectfully)" -- At 9, address it immediately the next time he acts disrespectfully and be firm about not having him come over to your house if it continues. (giving him a warning and an opportunity to correct the behavior). If you do terminate the work situation, make sure you let the parents know - this helps them to work with their son as well. Perhaps he will come back and apologize and continue to try -- which is definitely what we want to see the child do -- he needs to try even if he "fails".
Good luck -
S.