I think you're making a mistake, and I'll tell you why: formula, expressed breastmilk or cow's milk (whatever is in the bottle) IS FOOD. Your son is eating.
If he is drinking a great deal of juice, that will erode his appetite for other foods, as well as shorten him significantly. There is no human need for juice, certainly not undiluted, as it has terrifically-huge sugar content (even 'naturally' present sugar is sugar -- bodies don't care how it gets there). Even if it weren't for the nutritional zero (the same amount of vitamin c is found in 1/4 of a cup of tomato sauce, or 1/4 of a baked potato, so vitamin c is an easy nutrient to get way more than necessary)...
Even without the nutritional zero, the interruption of normal appetite, how much shorter the child will be than he's otherwise grow to, and the additional risk of cavities... even without all those negatives, juice still adds another risk: type 2 diabetes.
Juice provides *such* a huge hit of pure sugar (no fibre, no protein, no fat = instant absorption) that blood sugar levels go way up, then way down, then he wants more... do that all day for 4 years and the baby's pancreas is just worn right out. His muscles will become insulin resistant, and the pancreas just makes more and more insulin (because it stops working properly) and then quits.
Take him off the juice cycle by starting diluting the juice with more and more water every day until within a week or so there is a teaspoon of juice in a half cup of water, and then just eliminate it. His appetite will grow proportionally as he is delivered less and less sugar in a cup.
If fruit is going to remain a significant proportion of his diet, it needs to be whole, fresh or frozen, raw or cooked. But he'll have to get off the juice before he can be bothered chewing to get the calories he needs.