When he was born in Poland he would have been registered.
Well, he should have been registered. That doesn't mean that he was, or that the records survived the war. Many records were destroyed from the wars. There is an entire church archive that the Soviets grabbed twice, which has been pieced back together. Yes, it is important to find the family records somewhere in Poland, but he may need more than just his birth record. He may also need his grandfather's records due to the presumptions of the present law, and because he left before he turned 21. The application form does inquire about the grandparents as well.
You do not need to hire a lawyer or anyone else. This is a simple process,
No, in cases like this it frequently isn't simple. The process under the 2009 law presumes that even if he had been born a citizen, that he somehow must have lost it. In this case the OP's father was removed from his native country by the Nazis, and therefore he became a displaced person who had no Plish passport. There is a recent post on this topic stating that Nazis records of such deportations aren't accepted as proof of citizenship. The fact that he returned with a British passport may mean that the Polish government had no record of him. The process can require the petitioner to appear and present evidence proving holes that the administrator finds with the claim. Continuances are permitted to search for more documentation of the claim, and appeals are permitted as well. The recent poster who had a similar situtation won on appeal when additional documents were found in an archive of a brief return after the war. There are also the constitutional challenges as well since a person was either born a citizen, or not, under previous applicable law at birth regardless if he/she had applied for a passport before the 2009 law. Under the constitution, a person cannot be stripped of citizenship. The 2009 law, (which is really about recognising the rights of citizenship, i.e., passports, rather than citizenship itself), attempts to do that because someone in the ancestry chain refused to recognise the communist regime and register with it. Nothing in this process is simple for many people. A lawyer is highly recommended and the process can drag on for years.