Well lets see... If you tell the dealer it is for a 10 year old, he won't sell it to you... Or at least if he wants to keep his Yamaha Dealer's license, he won't. The major brands are supposed to be follwing the CPSC guidelines for engine size to age. That being said, I am in the same boat as you are. I have an 12 year old who is almost 6 ft, and 160. Good for football, not for riding a Raptor 80. I have prety much decided on a Honda 250 with reverse. It is a full sized machine... The new Yamaha Blaster is the actual size frame I like, but that machine is a full tilt racer... and not well suited to my son's rather docile nature. The Honda is much more subdued performance wise. Of course, he will be limited to riding on private land until he turns 16 unless we take the Raptor 80 along. Sure, he could lie about his age.. but then what sort of message would that be sending to him? I am teaching him to be honest ALL the time. Not just when it suits the situation. You have to live with that choice.
If your son was any younger, I'd say the Warrior is too big. But owning one, I also know they are reliable, and will serve him for many years to come if he takes care of it. Just remember to check the local state laws. You may be getting yourself into some hot water by putting him on a larger machine. Some states forbid any rider under 12.. and others are looking at 16 now.
I do not agree with the CPSC guidelines here. I would like to see 12 to 16 go to a 125 limit, and see manufacturers build something about the size of a blaster, but with the 125 on it. That is perfect world though...