Why is my dog so submissive?

I have a 9 month old puppy he is a Blue Heeler/ corgi mix and is on the smaller side. his shoulder is JUST past my knee. I went out of my way to socialize him when he was little, as dogs need to be well behaved on the base i live on. And for some reason lately hes not as feisty as he usually is, for some reason as he got older he is more submissive with dogs at the dog park. And either ignores them totally and just runs around, or runs from them. He used to be so vibrant when he was little, and jumped all over other dogs, and hes not sick i took him to the vet worriedly so what could it be? Is it since hes older, other dogs dont put up with his puppiness as much anymore? or is it something else?