We can continue to feed baby breastmilk and its not necessary to give any formula at all if baby wants breastmilk.
You can read more at kelly mom for more info: http://kellymom.com/category/ages/after12mo/
Breastmilk's nutrients changes to suit baby's needs as he grows. Its nature's perfect gift for baby.
I'm still feeding baby breastmilk now and my son's 13month old. I heard a lot of advices like breastmilk has no benefits after 6 months, or after 9 months of after 12months etc etc... but all these are myths with no scientific basis.