What might cause frequent urination in men?

Many things. For young men caffeine consumption is often the culprit. caffeine is a diuretic and signals the kidneys produce more urine they would normally. some men simply have small bladder capacity. for older men enlargement of the prostate is often to blame. also urinary tract infections, in particular prostatitis, can be an issue.
Infection. UTI, prostatitis, cystitis, and infection of the urinary tract. Diabetes should be ruled out. Get a urine analysis and culture and if sexually active and unprotected, get an STD screen. Abstain sexually for 1 week until cleared by your physician.