DSS is getting his first pony this week so he will need a saddle. I think the abetta will fit this pony and we have one in a 17" we can try to make sure its close. We need help figuring out the seat size though. DSS is about 53" tall and 53 lb. He wears an 8 slim in pants and needs a belt. They are getting a hair short on him though. He’s 9 and grows pretty slowly.
maybe a 13" or 14" western saddle for him? I am 4’11" (so 58 or 59 inches) and i weight about 130 lbs. I use a size 14" saddle. Your DSS is what…4’5"? You also want to make sure the stirrups will go short enough for him to be comfortable. Unlike the english leathers, you do not have as much adjustability due to the fender on a western saddle. You also can’t roll them, like you can with an English saddle 
Wintec makes a great youth saddle and it has a rounded skirt which will fit a pony better.
I won’t buy a wintec because they don’t honor their warranties without a huge fight. They had my bates for a year claiming it wasn’t actually broken, then they said it wasn’t broken enough to bother fixing, then they finally fixed it and stapled the panels on way crooked and flat refused to fix them. I had to have that done on my own dime. Abetta makes a round skirt in a 13" and a 14" just not sure which size to buy.
Seems like about now until Christmas, our Craiglist regularly has child sized, nice, older western saddles for sale.
You may want to look for those.
A size 14" should last a growing boy some years yet, as long as the stirrups are short enough now but long enough for growth.
13" Fabtron youth saddle is pretty adjustable in stirrup length, you can get them pretty dang short and they will adjust out longer as he grows.
