Vets often forget that it's really just best to treat rescue kitties for internal parasites.

They always think the shelters did - and they might have! But often the follow-up wasn't done. Round worm requires a series of
three treatments, whatever your vet thinks best to use for it. Each treatment needs to be three weeks apart. This is because no matter WHAT dewormer your vet opts to use, none kill anything but the adult worms. So you have to allow for the life cycle, and it's best, in the long run, just to assume that the second treatment didn't get all of them.
To prevent your other kitties from getting it, just scoop as frequently as possible.

The more often you can scoop, the less likely anyone will have a problem with them. If you want to be ultra sure, after the first treatment, toss all the litter, bleach the boxes, and fill them with new litter, and then keep scooping as often as you can until he's done at least the second treatment, if not the third. Then things should get back to normal.

Typical treatments are either Revolution (topical), Panacur, or Strongid-T. If they recommend drontal, my recommendation is not to use it. Around here there's a strain of round worm resistant to it. It was, for us, a waste of time and money and a LOT of frustration.