When can I start using hair conditioner to my baby?

My two year eight month having constipation problems. The problem started when he was one year and three months old. We approached 3-4 doctors they suggested to give him Fibre rich foods so we started with oats, resin, banana, boiled rice but none of this worked for him. He cries a lot whole passing the motion. He drinks milk and half litre per day. Is that is the reason for constipation? But I don't want to avoid milk. Tell me some solution.

At what age can I start using spices in my baby's diet?

How do I get my toddler to stop eating hair?