rogue,
My "quick" answer is that he shouldn't be riding on the tractor at all!
Now, my more practical answer is this. Do not have him anywhere near the tractor while you are actually doing work with it: mowing, using the loader, etc. After the job is done, if you wish to give him a ride, then belt yourself in, hold onto him tightly, and then cautiously give him a ride.
I say this for several reasons: First of all, young children have much more sensitive hearing then us older folk, and I don't think exposing him to the continuing drone of the mower is doing him any good. (Heck, I wear hearing protection myself while mowing!) Second, if you're busy watching the ground while mowing, you can't devote full attention to your son. An accident only takes a second. Third, if he does squirm out of your grasp into the path of the tractor, at least the mower blades aren't spinning.
Personally, I don't let my kids anywhere near the "big tractor" (
B2910) when it's running. They can sit on it when it's safely parked in the garage, but they stay clear if it's running. I do give them tractor rides on my small Honda lawn tractor (600 lbs. or so) AFTER I'm done mowing, for the reasons mentioned above. Our property only has a slight slope so the chance of a rollover is just about nil. However, even if it should happen, I'm reasonably confident I could handle it due to it's low weight. I don't know if I'd be so confident with a 1500-2000 lb. tractor.
Just my opinions, but after reading about many of the accidents posted under the "Safety" group, I feel a comfortable level of safety with my approach. It's an individual decision, you just have to be ready to live with the consequences. These things are not toys. Treat 'em with respect and you should be fine.
~Rick