Main drain leak

[Frogmom]

My pool drains about 1.5-2 inches a night visual and bucket test. Pool guy found leak behind light which he said he repaired. Still leaking at the same rate. Currently not below light yet but below everything else. Going to try dye at drain. No evidence at light yet. Leak is really slow because each dye test so far shows nothing. Once below the light I will bucket test again.
IF the main drain is the problem, how can I fix it without busting up the pool? I prefer not plugging the main drain permanently, but I don't really want to,bust up the concrete.

Thanks so much for your help.