Does anyone know how I can obtain a copy of his birth certificate so I can find out my grandmothers name.
You could start with Poznań Project: Search for the marriages database
, poznan-project.psnc.pl/search.php#search. It covers the areas of former Posen Province of Prussia, today's Greater Poland Province, with surrounding districts. This includes Złotów. Read more about this project here:
Their database of marriages covers the years 1800-1899, so it definitely does not apply to your father (born in 1925) but it could by chance apply to your grandfather if he was married in late 1880s.
The search page shows two interfaces:
1. Simple surname search, where you could specify just one surname Grochowski. Do not use it. It shows too many hits, so you are asked to use extended interface.
2. Extended search
You have to specify at least one surname (bride or groom), and optionally select a subregion, time frame, religion, first name. I tried these criteria:
+ Groom: Grochowski
+ Region: Bydgoszcz - Wyrzysk - Złotów
+ Time frame: From 1880 to 1899
And was provided with 24 exact matches - all 100% groom matches, and a bunch of approximate matches. However, no Andrjez (Andrzej) or any such variation of a given name was noted. The closest ones were Adalbertus and Antonius.
It looks like I found no match, but you may want to verify my search or extend the time range and try again.