A.H.
Are you able to call and speak with the doctor to clarify his recommendations/diagnosis? Do you have access to an ask-a-nurse/physician hotline? Many large local hospitals offer this service. You may be able to get a second opinion that way. I don't understand the no orange juice either. Maybe the citrus is irritating, but I would think he would say to eliminate other citrusy/acidic foods.
My son complain of this a few weeks ago. I told him to drink lots of water because sometimes when urine is really strong (dark yellow color) it can be painful to release. I also got him some cranberry juice because that seems to have some healing properties for bladder-related issues. It seemed to do the trick for him.
In regards to collecting a stool sample, our doctor said to place a paper towel over the toilet before your son goes because it makes it easier to collect and won't get diluted by the stool water. I hope you get some answers soon and the problem clears up quickly.Especially in time for his birthday! Poor guy!
HTH,
A.