It could either be blue balls as Shivam said.
Or it could be due to excessive fapping, his body is producing more vitamins/seminal fluid/sperm/etc. And so when he stops, his body still produces that at an excessive rate. After some time the body should lower the amount it produces and he should be okay. Just dont fap as the body will continue as it is and its depleting his nutrients which may cause issues in the future. He should also check for prostate enlargement or pelvic floor dysfunction.
Hope this helps!