I would also tend to agree to start them out with a full-sized hawk. Just not one of those half-axe heavy ones. Those small "mouse hawks" are fine for little kids, but they will need to get used to a regular sized hawk at some point.
And also go with a less expensive one. They can take a beating, and will be good to learn with. Just make sure it uses a standard handle size that you can get. The handles take most of the beatings, especially when the kids start playing "handles" matches/games.
But be a little careful with some of the cheapo's. Crazy Crow imports a lot of them, and are also a major wholesaler to most vendors you see at events. Some of their imports are "hand forged", but have problems inside them. The blades were wrapped around an eye mandrel, and fully forged out to that classic hawk blade shape. But then the two halves are ... arc welded ... together and ground to clean them up. Yeah, they went to all the trouble to forge out the correct shape on both sides, wrap it around an eye mandrel, but then didn't forge-weld the two parts together. So in use those two halves have a tendency to crack and split apart - especially if you resharpen the cutting edge a few times. You end up grinding off the arc-weld holding it together. If you pop the handle out and look down inside the eye, you can tell if it was arc-welded. That welding won't be ground smooth inside.
Now, they still are good "beater" hawks - especially for the price. They just won't hold up in the long run - or with heavy use.
Once your son gets some use and experience with hawk throwing, then it would be time to see about investing in a better one.
And don't automatically disregard Cast hawk heads. There are many really good ones out there. They are not cast iron, but cast STEEL. And the quality of that cast steel is usually pretty good. I have several that have held up quite well with the beatings they have taken. Even the couple factory "seconds" I picked up years ago have held up well. A little careful filing/grinding cleaned up the few problem areas from the original casting.
Just make sure you have a source of replacement handles that fit whatever hawk you choose. That will end up being the biggest concern over time.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands