Are you using vitamin D drops?

Ra
I'm from the UK where guidelines are slightly different from the USA. Now my baby is on 3 solid meals he takes around 18oz of formula as recommended. However now I've to use cows milk in his breakfast cereal/porridge meaning some days he may only now take 15oz of formula. In the U.K milk isn't fortified with vitamin D and the guidelines recommend breastfed babies and babies consuming less than 18oz of formula need vitamin D supplement. Should I give this to my baby? Would it harm him if he got to much vitamin D?