How old is your son?
Is he big/small for his age?
How physically adept is he?
I started my kids (at age 8 and 5 respectively) riding on their own.
It was a little electric quad, but it had enough power for me to burn donuts in the drive and was a great starter for my kids to learn steering control and tipping points.
Then we moved them up to a Suzuki 90 the next year.
That was a lot more power, but still maxed out at about 30MPH after we took the restrictors out. That little quad hauled my fat butt over the mountains a few times and was a lot of fun.
Then the next year we moved my older son (10 then) to a Raptor 250.
That was a real blast and gave him the ability to keep up with anybody on trail. Within a year he was jumping it as far as he could fling it. so by the time he turned 12 he was more than ready for a YFZ450. He was pushing the 250 beyond it's limits and needed the more capable YFZ suspension.
At 13 my older boy was racing his 450 in the ITP Quadcross. While his younger brother (10) is perfectly content to trail around on the Raptor 250.
Point being, by giving them the tools to advance their skills at their own pace both of my boys have become VERY competent riders and I have no fears letting either one of them ride my Raptor 700 as they have both shown years of experience when it comes to control and ability.
Personally, I would never ride two up for anything more than just putting around a parking lot in 1st gear. The quads that are designed for two-up have stretched wheelbases and heavy springs to keep them stable.