My baby's healthcare provider recommends give her rice and oatmeal cereal first in 4-6 months. And introduce fruit or vegetable juice until she is at least 6 months old.
Mix juice with water (1:1). Offer it from a cup, not a bottle when she finish breastfeed. No more 60-120 c.c per day.
If your babe loose stools, give him less juice to see if it become better or not.