There is no question of any consideration amount in the proposed gift deed. If any consideration amount is paid then the gift deed becomes invalid.
In fact if the donee pays the stamp duty, it can amount to consideration paid by the donee for the gift under the Transfer of Property Act. Since there cannot be a consideration in a gift deed, the deed may be considered void when the donee pays the stamp duty. So, the donor is responsible for paying the stamp duty.
The mother of the minor son can accept the property transferred through a gift deed to her minor son in the capacity of the guardian and next friend of the minor child.
Hence the stamp duty paid by the mother/ donor in the capacity of donor cannot be considered as a consideration amount towards payment of stamp duty.