I forgot to mention that they were Jews.
There was a Jewish cemetary in Grajewo on ul. Bialystocka, but has now been built over but there is a plaque ul Majowa in memory of those who were killed by the Germans - this includes Poles, Jews & Russians. Some say over 5,000 died from there during the war.
I am not saying that he was one of them, but this may help you eliminate possibilities.
You may one of these may have more details for you:-
Taube Foundation for Jewish Life & Culture
Jewish Civilisation Administration.
Grajewo is in the Podlaski area.
If you come over any Polish words, I'll be happy to help
My late grandfather was born in 1911 in Grajewo, Lomza, left Poland in 1928
I've just realised that Grajewo was handed over to the Russians by the Germans in 1939. So I've done a bit of research.
Lots of its residents were sent off to Siberia (somewhere near the White Sea referred to as far north to the 'White Bears' ). Mainly the were individuals in the army, police, foresters, intelligence service.
The second tranport consisted of the families of those who had gone on the first transport as well as administrators and richer families. They too were sent to the above.
The third wave of tranport went to Kazachstan. Mainly families of the intelligentia, wealthy farmers, administrators, buyers and tradesmen.
With the commencement of the Russian German war, the Germans entered Grajewo on 22 June 1941. In 1943 the Germans killed around 150 of Grajewo population referred to in my previous.
This is a summary of information which I obtained from a Polish website in Grajewo: