Quoting Mama MacGyver*:" Make sure you DON'T give him bananas, cheese, or applesauce either. Those will constipate him more!"
YES! THIS! Plus you MIGHT want to limit milk until his poop is better. My 4 y/o has been prone to constipation since about 2 and i keep an eye on her poop daily. When she has lots of ANY dairy, she struggles a bit.
The juices will help at full strength. dont dilute.
If he doesnt want juice, try berries and grapes and raisins.
If the problem doesnt get better, i believe there is childrens colace that will help soften the poop so it can get out. Hopefully, the poop isnt shaped and hardened like a golfball at this point. If that is the case, PediLax liquid suppositories WILL help. But that is if changes in what he eats doesnt work.