A. The most common condition causing this symptom in Chihuahuas is patelar luxation - basically it's the knee cap going out of place,
It's not always advise to opt to surgeries in these conditions as it is not always a painful condition, it just causes a limp in some cases.
I advise taking him to the vet in order to confirm the possible diagnosis and decide together with him what is the best thing to do.
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