I think the manuals say that no one under the age of 14 should be using these tractors. I wonder with all the safety switches on them, but I have seen hospital statistics that are very high on kids getting hurt on them. My kids have been very unhappy with me that I made them wait until they were at least 14 to mow. I did let one of my sons pull a trailer around some when he was turning 14 so he could get used to it. No mowing though.
I had a boss who taught driver safety to BFI drivers who always said there are no accidents, only people dropping the ball. I never wanted to see my kids in the emergency room, and mower accidents are not pretty. My sister-in-law had her foot nearly cut off by one when she was about the same age as your son.
If you think I am a prude, I had my oldest sons splitting logs with a Monster Maul when they were 12. But I taught them the right way to "throw" a maul so that if it slipped or they missed it couldn't come back at them, and made sure they knew if they were tired to quit.
These things are not toys, they are fun, but can be dangerous. I won't even mow if there is no one home.
One of my 23yo daughter's friends was helping his Dad cut wood when he was 13. His Dad cut his son's hand in half. They found all but one finger so he is missing a finger now. 2 weeks later the Dad picked up the saw and the finger fell out. It only takes a second to change a life forever.
Since you asked, it makes me wonder if you are really comfortable with this, or if some instinct is steering you away from it. I would really suggest that you wait. There is a BIG leap in maturity between 10 and 14. There will be plenty of time for him to mow when he's old enough to think things through. His physical strength and reflexes will be better developed then, as well. Good luck.