Unless you can find the original manufacturer of that specific chain, you’ll probably have to settle for something similar but not exact. Most in the trade would probably call that an Alternating Mariner or Anchor chain, but neither one is 100% accurate.
It may be helpful if your nephew can recall where it was purchased. It may have been from a jewelry chain like Zales or Kay, or even from a department store, many of which have their own proprietary lines. If it was from an independent jeweler, they will most likely remember who they got it from and would be able to replace it.
Most chain manufacturers like Quality Gold or Royal Chain (used to be Leslie’s and Aurafin), or Downeast Trading Company have things that are similar, but each has their own slight variations. You’ll need to get an account with any of these companies to buy from them though, or find someone that has one.
If you do find a suitable replacement, make sure you solder those jump rings closed before you let him wear it.
Not very helpful, I know. Sorry.