Here's my thoughts:
Conventional reels are a compromise. Compared to spinners of the same weight, you get better line capacity, better casting distance, and ability to land bigger fish. In exchange they are harder to cast on a good day, even if you are just gently lobbing dense, aerodynamic baits with a 7 foot Ugly Stik, with a tailwind.
Make it a 9 foot surf rod, with the added distance expectation per cast and a likely headwind, and the learning curve gets much bigger with a conventional. Your son could certainly get the hang of it with practice, but it would be at the cost of his enjoyment.
A 9 foot surf rod is medium-large-size spinning reel territory. A decent 4000 - 5000-sized spinner, loaded with 15 - 20 pound braid, will handle anything within reason in the surf zone. It would be easier to cast, and in terms of casting distance, the reel would not be the limiting factor, the rod would.
If it were me, I'd get him a Tica or Penn or Tsunami or Daiwa 8ft or 9ft rod rated for up to 3 oz. Let him handle a few and pick one. Then throw a daiwa black gold 20 or SS 1600 on it, and watch him crush fish to the first bar for years.
Then, when he's big enough to load a bigger rod, he'll still have a damn good bait catching setup!
