Hey there, not usually, unless in the rare instance the vaectomy has failed, they also suggest you dont have sex right after it has been done either because there is still a chance you can get pregnant. I hope this helps :)
The man can get his sperm count tested at the doctor's. In fact, after a vasectomy they do this (like, about three months later) to be sure the count is down to zero. If the man never had this test, there is a possibility the vasectomy wasn't effective in the first place. And as GA says, there is a slight chance that the ends of the tubes can find each other or sperm can make it past the cut. Easiest way to know is to go to the doctor and ask for a sperm test.
The chances are slim but yes it IS possible. The tubes that are cut during a vasectomy can grow back together or sperm can manage to get through and mix with the semen. The only PERMANENT birth control is abstinence.