It is tall, I’ve gotten the seat a couple inches lower than stock but still can’t flat foot it.I might have been but is it really a 35" seat height? https://www.kawasaki.com/products/2020-KLX250?cm_re=MPP-_-PRODUCTTRIMLIST-_-VEHICLEDETAILS
If so, that wouldn't work. He's not tall enough for that and still very much a 'new' rider.
Not sure a TW would work as it could ultimately be a school commuter (though I break out in hives thinking about that).
Whats his height and weight?