Hello Chestergirl
It's hard to say on the basis of your post but I would say a) he may have been a forced labourer in Nazi Germany, or a camp inmate, even at his tender age; b) he was very probably an officially registered Displaced Person at the end of the WW2, along with hundreds of thousands of others; c) he did not wish to go back to what had become Communist Poland, or believed he had no surviving family there; d) he wanted a new life somewhere in the West; e) Australia, like Britain, accepted many thousands of DPs.
Are you sure he had a full British passport, rather than temporary travel documents issued in UK? It's somewhat unlikely he could have got a full GB passport within two years of arrival in Britain.
Feel free to PM me if you wish.
Bluebird