He should see an internal medicine doctor
Anatomic problems need to be ruled out...such as small bladder...and he needs an ultrasound for this.
In some cases there can be not enough anti-diuretic hormone and a drug called vasopression can be used to replace this.
If no organic problems are found like this, then he needs to see a urologist to look at bladder functioning, get the correct exercises to correct any problems.
He might instead need a sleep study....as he might have a sleep defect in which he does not wake up to normal stimuli.
These would be the main possibilities other than psychological.....
Time that he was examined.
Let me know if you have further questions, details or need clarification, just use reply.......
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