J.S.
I'm dealing with constipation with my 8 month old and we are controling it without medication (which I didn't want to give her). We give her prunes and pear juice - I know you can't give a 3 week old prunes - but I'm pretty sure you can add like 2 Tablespoons of pear juice to his bottle. I'd ask the doctor, but that's what my doctor recommended to us when our baby girl was really young. He said that the pear juice has more fiber in it and actually works better than prune juice (probably tastes better too!).
Hopefully this helps. I know I feel helpless when my little girl is trying to go and she starts crying. Good luck!
J.