Hi Mike, welcome to the forum!
Since I owned, or at least rode, almost every complete on the market, during the R&D time of our own ESK8 brands, I would like to share my experiences with you.
First thing you need to decide is what kind of board your son would enjoy to ride most. You have basically 2 categories with 2 sub categories. You can choose between an AT board and a street board. Since your budget is limited to 1k you need to stuck with Chinese street boards. To get at least a minimum amount of service and warranty, I would go with the big 4 (Meepo, Exway, Wowgo, Backfire).
Next step is to decide if you want to go with belt or hub drive.
All 4 brands have now both options available. So let’s see what we get.
Exway Flex
The Flex is a high quality budget board with awesome trucks. Their new Trist trucks are the only forged trucks you can get from those brands. The board is also the only one with an additional App and comes with a smart battery. You can turn the board on and off by remote. Third cool thing is that you can choose between belt or hub drive. Those are all unique features. Range, speed and acceleration is similar to all other boards in this category. What you really ne to consider is the deck. It is very flex and has a very aggressive concave. So if your so is heavy (over 200lbs) and/or has large feet, it might not be the right board for him. Same thing if he prefers a more stable and less playful board. Price wise it is the cheapest of those boards.
Meepo NLS PRO/Belt
The NLS is a reliable and fun board to ride. It has a flexy deck, strong/reliable hub motors, a high top speed (hub version) and an average range. The 100mm wheels and the flex deck makes it a quite comfy board. It has all basic features, nothing special (besides push to start) and I would say it’s an solid over all board. Since the belt version is very new, I didn’t hat the chance to test ride it yet. Anyways, I wouldn’t recommend getting the belt drive, at least not with stock wheels, since it is low on top speed (might be a safety feature for your son). The wheel quallity is also not proven. A youtuber had lost a big chunk out of a wheel after the first couple rides. The range and the price are similar to the other boards. Keep in mind you will get with the belt driver a better acceleration, than with the hubs.
Backfire G3/Zelot
The backfire is also a more premium approche of a skateboard comparable to Exway. It is again a very flexy deck with average acceleration, top speed, range and good brakes. I prefer the deck over the Exway since it is a little wider and has less concave. It has all basic features you need, but nothing special. I need to mention that some customers had problems with the hub motors but nothing unheard of. The price is also average, the Zealot being a little pricier.
Last but not least the Wowogo 3/3x (8Ah)
The wowgo is arguably the most basic board here.
It is a booted clone like all other boards above and it has no special features. But it has very nice Paris trucks though! Acceleration, top speed, range and price are again comparable to the others, while the belt version is a little bit more expensive but on par with Backfire. The deck is again very flexy and you’ll get all the basic features.
So how to choose the right board now, since they are all very similar? If your are has rough terrain I would probably go with a belt board. You have more opportunities wheel wise and you will get a smoother ride. If you want to get the benefits of less maintenance, a quiet, low profile and a sleeker looking board take the a hub version.
If you can’t decide go with Exway and get both.
My favorite would be the Exway 2in1 with the Backfire deck.
But also the NLS PRO with the Wowgo/Paris trucks could be a nice combination.
I hope I could help you a little further.