It's a flag for diabetes. The term diabetes insipidus comes from the Greek word for "fountain". As thyroidian suggested it could also be prostate, but he is a bit young.
He needs an hba1c test.
how long has this been going on? has he had his glucose tested?
could be a number of things go to docs for blood test could be infection or a form of diabeties
Hi frquent urination in a man at night, could be from a couple of causes, being the prostrate is the most commom.
You also urinate alot if the detrusor muscle is overactive, or you suffer with diabetes,etc.
My first guess is this could be preostrate related, and your boyfriend needs to go and have this checked out, with an Urologist, a consultant.
Hope this helps
PS there is medication to take to help, but you need to find the cause first,
Good luck
Nicola