Join the club
The Frustrated Mothers of Boys Who Ride club that is. There is definitely pink and purple overload in Pony Land. As for show clothes it is slim pickins’ out there.
At 7 my son wore unisex jodphurs, paddock boots, white dress shirt and tie from the boys dept and navy suit jacket from the thrift store. That would be fine for local, unrated showing.
By about 9 he started wearing paddock boots and half chaps but the top half was still not specialty riding wear. Not sure about the US but you can show rated in Canada with half chaps. Do not recommend buying tall boots for a rapidly growing youngster until you absolutely have to. I even buy used paddock boots when I can, I got used Ariats (size 7 womens) for my now 11 yr old son in March, they are now on the verge of small. The half chaps that fit in March are also on the snug side now.
Since he is now 11 I am on the hunt for proper boys breeches (back pockets) that come in size 26 waist from an online retailer that ships to Canada for something less then an arm and a leg. Ovation makes a boys breech in that size. Mens breeches start at size 28 waist.
I am also looking for a boys show jacket. A girls show jacket no longer works as by this age the jackets start to have some shape. Dover make a boys show jacket but again, the shipping and duty make it ridiculous. Ovation makes one too, apparently.
Unless you are doing the leadline at Devon, the jacket you bought for the wedding will be just fine.
Good luck!