Any appropriately sized simple paddle fin. Look for something similar in design to the Mares Quattro or Oceanic Viper, though it doesn't have to be those specific models. Don't bother with any expensive gimmicks involving extra plastic pieces like "power bands" or "folding fins" etc. Do NOT get "split fins" whatever you do.
Probably best to go for the open heel + neoprene boot type. These are warmer, offer better protection, and the boots are useful as general purpose tough water shoes. Unless he only ever goes in very warm tropical water - then you might be okay with closed heel bootless fins. If you go for the closed heel style, opt for a size bigger plus a pair of 3mm neoprene socks to prevent blisters.
Don't spend too much because at that age he WILL outgrow them very quickly.
Go to a local dive shop to try some on and get advice, they'll likely match internet prices and it's vital to support those businesses.
Bright colored fins (especially white or neon yellow) will make him much easier to see in the water.
If you're looking to save a bunch, look for used fins on ebay/craigslist/fb marketplace. Fins don't really expire (other than maybe needing new rubber straps) but they depreciate in price a lot. One of the best pieces of gear to buy used.
long freediving fins, even cheap bad ones, are also super fun for kids because they make you go REALLY FAST.