L.L.
J.,
As a mother of four raising 3 grandchildren I can only tell you my thoughts from experience. I also have a severe stomach disorder.
Milk is not all that it is cracked up to be these days. They are finding it can actually cause harm to children because of the added hormones. My 7 year old started developing at the age of 7. I took her to the dr. and this is what he told me was the cause of the early development. When I took custody at the age of 2yrs.10mos. she was only drinking milk with Karo syrup in it.
I think it is a wonderful thing that your son likes to drink water. Actually, water will be better for his bowels than anything. The juice can cause tooth decay but water is perfect in every aspect. There are other ways you can give him calcium. You may want to check the internet of dr. as to what will help him get the calcium and vitamins he would get from the milk.
I hope this helps in some way.
L.