I don't know shit about them, but I'd start cheap and small... His skills will likely develop quickly and in a few seasons you'll be replacing it with something else anyway. And, there's a chance (however remote) that he won't be incredibly into it... Power-wise, 50-80cc will do just fine based on my dirt bike experience... if he's a gear head, you may want to start smaller (or don't let him see where the throttle stop screw is)... Physical size is going to dictate a lot, despite similar engines sizes, ergos can be vastly different... As far as all that other shit is concerned, I probably wouldn't bother to narrow it down that far... If you stick with a brand you heard of, generally those small machines are reliable and simple to maintain.

 

You can get a Polaris Predator 90 for kids. It comes with a rip cord for both the kid and the parent. The kids one shuts it off if they come off. The parents one attaches to the back and you usually attach it to your 4 wheeler and if they get out of range it pulls it and shuts them off. As well you can adjust the power on it so they can "grow" into it.

Polaris makes some of the best kids wheelers. If ya want more help with it message me and I can help ya out.

 

My buddy had an older polaris 90cc two-stroke for his kid. It would do almost 30 with me on it, pretty fast, when it was actually running. But being a polaris that wasn't very often.

Kids quads are all over craigslist. People buy them, kid outgrows it in a couple years then its for sale. You can pick them up for a couple hundred bucks. This is one case where getting the chinese off-brand really isn't that bad of an idea. A four stroke 70 or 90 would probably be suitable. Some have remote kill switches, or shut off if it gets too far away from the remote. Trust me kids can and will figure out the little screw to restrict the throttle flipper.

If you want him to do things right and not grow up to be a pussy ass quad rider, get him a dirt bike. Create a future motorcyclist. Kids love to have a little bike parked next to daddy's big bike. Little 50/70cc pit bikes are everywhere also.

 

Finding quite a few 90cc 2 strokes in the $100-$150 range. Some are runners without tires, seat and body work. Others are complete except for a motor. Thinking I can pick up one of each and him and i can throw the motor into the good chassis. I can't imagine a motor swap being that hard on one of these

I did find a couple complete running 50's and 80's, but they are running $700 and up. More than I want to spend on his first one

 

I would suggest a chineese knock off for a first bike. LOCTITE everything on it though. If your son likes it, great upgrade him to a honda/ kaw/ yam ect. when he grows out of it. Thats what I did with my kids. It was a semi inexpensive way to find out only 1 of the 3 actually enjoyed quads and when it was time to sell I didnt lose any money. also I was able to jump from an 80 to a Raptor 350 as my sons second bike.

 

The chinese ones really aren't that bad, at least for children to use. As mentioned give it a once over for loose bolts and loctite as needed. The motors and stuff are pretty decent, since they're virtually exact copies of Honda or Yam engines, some can even interchange parts they're so similar.

Especially if you're not sure he'll get into it, if not then you're not out that much money. People might get $800 for a good shape name brand one, you can buy a brand new chinese one for like $400 and used ones for half that. I see listings constantly for them "been sitting, won't start" for like $100. Clean the carb, big deal. Worst case replace the carb they're like $30 on ebay.