oh good question actually, this has an interesting answer
originally, he wore a sweater as a kid, you’ll know it as the “ova fit”
when he outgrew that, he’s had a few more as he grew into his adult self, the current one hes wearing was actually a gift
from blaze!
it was his old hoodie, he wore it for similar reasons, and he didn’t need it anymore, so he handed it over to sonic
it still fits