I would expect some discharge the first few days but you said it's been 12 days. I don't know how large the incision is. Did they harvest a vessel for bypass surgery? That would be a rather large incision.
If the oozing has been there all along and is still the same clear to pale yellow color, it's probably still in the normal range. If it's just started, is darker in color or bloody, if there's redness, swelling or any warmth to the area, he needs to be seen ASAP by the doctor.
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