According to iStocks Extended license agreement, it sounds like he can resell your image.

I copied the following from iStocks website...

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Items for Resale - Limited Run
You may purchase Extended Licenses allowing uses of the Content in items for resale, license, or other distribution, including:

up to 100,000 cards, stationery items, stickers, or paper products,
up to 10,000 posters, calendars, mugs, or mousepads,
or up to 2,000 t-shirts, apparel items, games, toys, entertainment goods, or framed artwork.

Electronic Items for Resale - Unlimited Run
You may also purchase the option to resell the Content in an unlimited number of electronic templates for e-greeting or similar cards, electronic templates for web or applications development, PowerPoint or Keynote templates, screensavers, and email or brochure templates.
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From the above, he has the right to resell your image as art work up to 2,000 times.

If you don't want this to occur, you can opt out your images from being available for extended licenses.

Joe