It would all depend on whether he was sentenced under the name you know him by. If he has been using alias(es), then that will be difficult.
Do you know if the people who visited him in prison, know his real name. I would suspect they do, as visiting someone in prison is not an easy thing to do. The security you go through to get inside is very strict and they need their passport to gain entry on most prisons (although I\'m not sure about an open one). So, they would have to know his real name to ask for vistation visits. Can you ask one of them, if the name they know him by is the same as the one he gave you.
If you can give any other details in regard to him, name, date of birth, N.I. number, where he says he comes from (which of course all might be lies), even a copy of his Driving Licence (only do be careful when trying to find these things out), that would give you a start. A photo is a must as well. If you can\'t find out his real name, even doing a search on an alias might yield results and failing that, I would suggest a Private Investigator is your next option.
Please come back if I can offer you more help?
:)