I'd ask how old is you son? And what type of riding will you do?
I recommend something like a 340 to 500 fan cooled for a new rider. Not much to go wrong and easy to work on... Smaller has some advantages for keeping their speed in check as well. For me I found a '98 Jag 340 that has been great to us. It would do 65mph no problem. I bought it figuring if something did happen it was not supper expensive. I also made it very clear that if something happen it would not be replaced right away. I also planned that if all went well, it would be passed on to my youngest. He will be 12 this November. So this past February I found a '16 ZR5000 that I picked up. My oldest is on that and youngest will be on the '98 Jag 340. If all goes well I will probably look to upgrade him to a 4 stroke in 3 years or so. The next sled will depend on what I find or if I'm ready to trade up from my '15 ZR9000. The ZR5000 is nice and reliable but without the power of the 7000 and up sleds. My oldest could handle more power but I don't want to temp him either. He figure out very quickly if he slowed down on the straight-a-ways by the time I noticed he could speed up and go faster than my pace to catch up and than wait for the next straight section to do it again. He's already complaining we are going to have to go slower due to his brother riding this year.
One other thing to note the older smaller sleds fit the younger kids well, but they can grow out of them fast. My oldest needed a larger sled to fit him better which was part of the reason I bought the ZR5000. He's 6'3" at 14 years old.
Good luck