Six days is a lot. If this becomes regular have to consider to give him more plain water or change formula. Bm babies should pooped more easily. Be careful as constipation can increase risk of UTI which happened to my gal. I changed her milk powder and it became better.
If he is on solids can give him some water to drink already. Eating solids can make them constipated as their tummy get used to new diet. Can give fruits starting with P, pear, papaya, plum.
For first solids you can try mash avocado or white rice cereal.
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