Yeesh, doesn't look too bad so far, but not good either.I've seen a progressed sinus infection in a pigeon before and it did not end well. There are a lot of nerves in the avian sinus cavity and even if funds aren't an issue, you'll likely have trouble finding a competent vet willing to do surgery due to the complexity of the area if it gets that far.
Poultry respond to infection the same way throughout their body - make a hard kernel, force it out. Which is why we get the famed bumblefoot kernel and hard rubbery lash eggs.
Now imagine a hard knot forming in those sensitive sinuses.
I do think antibiotic and early is the way to go, personally (though I am not a vet). I would want any infection cleared out before it gets the chance to form a hard kernel.