You should start by obtaining records in England........Grandparents marriage record, birth records for their children, church records, etc. Hopefully, by obtaining these records, you will find out his birth town information.
You can obtain an England Research Outline by going to www. familysearch. org (No Spaces), then to the "library" tab, then to "education." Click on "Family History Library Publications, then "research outlines & then scroll down to England.
Hope this helps.
JCSM has some good tips.
ALSO try to find these answers: what region was he from in Poland? Does he recall anyone's name or dob, village or occupation? Do you know if the spelling is accurate for his last name? Get all of this info together before you begin.
If he believes any relatives went to Canada or the US, there are various places to start looking there. ICE and Nara and the Census bureau all have records not generally available to the public that, as next of kin, you may be able to get.
If he was Jewish, there are specific online genealogy groups for you to start with. And in Poland, if he believes they ended up in a camp and may have survived the war, Red Cross and similar records now available to you may help him to find where they ended up.
Have you checked the Genealogy resources here, at the top of the list of postings?