I don't believe Leukeimia is a side effect of MM...I've never heard of such a thing. Instead, it's a possible side effect of chemo. Nearly every disclaimer I signed prior to receiving chemo stated the possible danger of developing leukeimia and other secondary cancers.
It doesn't sound like it has been that long since your father started treatment. I would suspect either a drug reaction is causing the drop in blood counts; or, his myeloma is back.
But I'm just a patient, so hopefully your doctors can figure it out soon.
Best of luck