How Much Juice And What Kind Of Juice Should I
He does not need any juice for nutrition. And, he does not need treatment
He does not need any juice for nutrition. And, he does not need treatment for constipation unless his stools are hard, round, dry balls.
If he is not pooping frequently, but what does come out is soft, then this is just normal. You may even have a "blow out" poop where it is everywhere, even coming out the diaper up his back. If he is constipated, you'll need about 2 ounces of prune juice, and you'll need to dilute it.
You might first try Pedialyte, which often works because of the added electrolytes and is safe.
Some amount of gas is normal for babies because their intestines and stomach enzymes are still maturing and adjusting to eating. If he has a fever, or blood in his stool, or is in true pain, then he needs to see a doctor for an evaluation.