3 yr old incontinent

I posted about a month ago about my 3 yr old son refusing to toilet train. We have discovered he seems to be incontinent and we are currently going through a heap of tests to find out why, (urine and blood samples) he drinks a lot of fluid at least 4 liters a day. Our dr thinks his drinking too much water and stretching his bladder then aparently a bladder that is always stretched can become incontinent. He also said it is very common in children that are alittle late with toilet training. The way to fix his stretched bladder is to limit his fluid intake to 1 litre a day which is the amount he needs for his age and body weight. Slowly the bladder will reduce to its normal size and the sensation to needing to pee will return.

Has anyone's child ever had this issue? How did they limit their fluid intake as my little boy keeps saying his thirsty and I feel bad his not aloud anymore liquid. Also how long did it take for the child's bladder to get back to normal size ?

Thanks from a very frustrated mummy