Get him a seatbelt! Teach him early.
My son has been on the tractor since he was 2,he’s about to turn 5. He can run the loader pretty efficiently, we’ve only had the back hoe just over a week, he got a little freaked out the first time I let him put the stabilizers out and he did one at a time, once the tractor shot up on one side he was done lol. He will give my wife rides on the zero turn too(she has no idea how to drive it).
Both my kids love everything outside and would rather be out there with me working, but I am cautious as to what they actually get to do (we do a lot of riding around). The other day I was on my way into town and there was a lady mowing on a zero turn with 3 kids on it