I’m confused as to why they would give you injectable antibiotics for an eye infection, that won’t do anything.
the picture you posted did not look anywhere near severe enough to need removed. Did they do an eye stain to confirm that it’s an ulcer? If the eye has not gotten worse after this much time, it’s likely not even infected, I would be willing to bet that something is still stuck in his eye where you can’t see it. When this happens, the result is an ulcer or abrasion that won’t heal. Can you clarify what eye drops you are using and the dose?
one thing about eyes is that the vet really doesn’t need to be specialized in guinea pigs to treat it, because eye issues are not specific to them.