Boys usually have no butt, so smaller seats will work better. Much will depend on how high the cantle is behind him. Lots of the high cantle saddles used for barrel racing now, so they don’t allow much movement by the kid while riding. If that high cantle saddle doesn’t fit exactly right, it is NOT fun to ride. Lower cantle saddle is very body adjustable, allowing movement for comfort, so will probably be rideable longer.
My son rode in a 12" seat, was in perfect body position (head, hips, heels aligned with no effort) for quite a while. He rode that saddle from about 6yrs on. He was not a big child, slender, and just got taller as he got older, no wider. I think I moved him to a 14" seat when he was almost 13yrs. His sister started riding in the smaller saddle at about 6yrs, but had more of a girly figure, had to move to the 14" seat at about 11yrs old. However that 14" seat saddle fit both the kids on thru their teen years and they won plenty riding in it. Just let them fall into good body position naturally, not slopping around in the seat.
I see a LOT of smaller children and kids riding in saddles that are WAY too big in the seat for them. Kid is just all over the place while riding. I see a lot of poor equitation, because those big seats don’t let kids get their legs under them, for a balanced ride. Take a look at your child in a small seat saddle, is his body lined up without him working at it? You want riding to be EASY for him at this young age, so help him with a good fitting saddle.
Those smaller seat saddles are available at reasonable prices. Buy what fits him NOW, so riding is fun. Trade up in size later, when he is larger. My kids were very confident riders, their saddles fit them, made riding easy. Even if they only use that saddle for a short time, it is worth the investment. I had 3 people jump to buy my small saddle when we sold it, almost a fight at the tack sale!! I made money reselling it, was a good brand name, well made.
I strongly advise AGAINST a saddle as big as the 14" at his young age. Let him try the saddles on if you doubt me, so YOU can see how they fit him now. You can buy a one inch larger seat if he does grow, 13" seats are also available if you look. Saves that big jump to 2" larger seat.
This big saddle seat thing is just numbers. There is no actual reason to get such big seats for little kids, except “everyone else does it” thinking. I saw the same thing in Pony Club where there was a constant push for bigger animals under the little kids. So weird because the little animals FIT THE KIDS. But it was a kind of silent pressure to “buy bigger” all the time. It wasn’t considered a “real horse” if it wasn’t over 15.2H. Again weird, since kids were skinny little things, not even 100 pounds and couldn’t MANAGE the big animals!! My kid didn’t get a bigger horse until he needed one for bigger heights he had tested up to in jumping. The old mare he started with had a 2ft limit, she was 29 yrs old when he finished with her! Did everything else just fine at her 14.2H size, and he fit her well. She was obedient and under control in all the activities he competed in, Games, 3-Day Eventing, Show Jumping Rallies. His Teams brought home good ribbons.
Don’t be pressured to “buy big” in saddles or horses. Buying and letting the kid “grow into it” has not worked well in my experience. Kid doesn’t have a good time, not comfortable whether talking saddles, boots, horse he rides, and often quits because there is no enjoyment or even comfort, riding.
Would YOU be comfortable or even safe, riding with nothing that was your correct size? Why do it to the kid?