I am sure there are a few reasons. One that comes to my mind though is anxiety. If he is preoccupied with a stressful event (previous or present) this could be the cause.
Urinary tract infection is another.
He could also have an enlarged prostate or a bladder infection. Ask the health care provider to give him a urinalysis, which can be given in the office, or get a referral to a Urologist and get checked out.
A feeling of needing to urinate frequently could have many different underlying causes, ranging from having had a lot to drink, to a urinary tract infection, to prostate inflammation/infection, to a sexually transmitted disease.
It's probably a good idea for your boyfriend to see his doctor to determine the root cause of his symptoms so they can be treated (if necessary).