Well, my sponsorships are through the Sheldrick Wildlife Trust, but they're a rehab and release facility for orphaned elephants and rhinos only so that's not going to help you. I googled "adopt a panda" and it came back with a bunch of options -- WWF, National Wildlife Foundation, Pandas International, World Animal Foundation, National Zoo, San Diego Zoo, adoptapanda.net (this is a UK affiliate of WWF but the prices look very reasonable). The cost to adopt and what you get in return varies quite a bit between these organizations so you'll have to figure out the best fit for you.
I will say, the elephant adoptions we did are fun because you got to read the backstories and pick a specific orphan by name and then people receive monthly email updates pertaining to their foster. People feel very invested in "their" elephant that way. If there's something comparable for pandas I would go with that. Unless your nephew is very young, then he might prefer one of the gift bag/receive a stuffed panda type of adoptions.