Also, "natural causes" could be euphemism for some combination of starvation, disease, exposure, and exhaustion, of which many, many people died in the camps in addition to the death chambers. Do you know which camp he was in? They had a great documentary on PBS at the time of the 60th anniversary of the liberation of Auschwitz. In it, they said that there were two camps at Treblinka, one was more of a pure death camp and the other a slave labor camp. Knowing which one he died in would probably give more insight into how he was killed.
I have found a page with the Jewish side of my family's geneology, by the way. My grandma was Ilse Hirsch, so by that reckoning my great grandfather died in Auschwitz (not Treblinka - I think I got it mixed up with Theresienstadt, where many other relatives of mine died).