Are Vitamin D drops necessary for exclusively breastfed babies?

Ashley • Mommy of 3 Landon (3) Alec (1) and baby Autumn (4 months)
My sons doctor has been telling me 9 months that vitamin d drops are needed because hes exclusively breastfed... it doesnt even make sense to me.... do they need the drops? My son is 9 months old now and has done just fine without them...