My 3 and half year-old son urinates frequently. He will just have gone to the toilet only to wee a few minutes later. He sometimes remembers to go most times but often not and will wet his pants. We have had him tested for diabetes insipidus but the results came back normal. He also under went an MRI of the brain. His fluid intake is normal. Any ideas of what the next step could be.
Your son has certainly had very sophisticated investigations for his urinary frequency. I presume he has had a test for urinary tract infection and diabetes mellitus. I would very much like you to inspect the tip of his penis. If he is uncircumcised push the foreskin back gently (DO NOT FORCE IT BACK) and see if there is any infected material under the foreskin. If he is circumcised look for a small ulcer adjacent to (and possibly running into) the hole at the tip. If either of these is present you need to see your doctor again.
Is he fearful of going to the toilet. All these ideas are to see if he is only letting out a small quantity of urine at each visit.
I would be interested in what happens.