Lactose intolerance can be congenital (then it is lifelong issue) or transient (following gut infection, in which case there is full recovery once the lining of Small intestines heal). There is Lactose-free cows' milk (from Amul available online) or even Lactose-free formula powder milk (usually indicated as LF) available in medical shops: can easily use these. Drs recommend to continue breast milk even if there is lactose-intolerance: but since your child is 1.5years, the suckling is likely to be non-nutritive & more for emotional-bonding of mother-child.